Buttigieg comes under fire from Warren and Klobuchar at 6th debate in California

11:53 p.m.: After Warren went after Buttigieg's high-dollar fundraisers — most recently one he held in a "wine cave" — during the debate, she told CBS News' Ed O'Keefe she is taking issue with "the kind of campaign" Buttigieg is running. Warren said she decided "not to do a business as usual campaign," and she said she started "this campaign by doing selfies and going to town halls." 

"Yesterday the President of the United States was impeached —  the basic issue is corruption," Warren said. "It's about how he's enriched himself, how he's enriched others, how he sold the ambassadorship. Democrats are gonna win from where we draw the sharpest distinction between Donald Trump and his corruption that is helping the wealthy and the well-connected —  and that we are going to be on the side of the American people. And how do you credibly draw that distinction? Well for me, it's partly about what you show by the kind of campaign you run."

Warren repeated that she doesn't "sell access to her time" to high-dollar donors. Although California Governor Gavin Newsom said earlier that the wine cave that Warren referenced is a place where many progressive Democrats have fundraised, Warren continued to push back on Buttigieg's fundraising there.

"It is the case that the mayor said a couple of weeks ago that he was not going to do any closed- door fundraisers and then he ends up in a wine cave filled with crystals — where they are drinking $900 bottles of wine and at that dinner, the press is locked out," Warren said.  

CBSN's Caitlin Huey-Burns noted to Warren that she and Klobuchar — the only two women on stage — were the only candidates who asked for forgiveness. 

"You know, look, all I can say is I know the kind of race I'm running and I know the kind of fight I'm fighting and I realize that sometimes I get a little hot when I do it, but it's not because I'm angry at somebody It's because I'm angry at a system that's broken," Warren said. "A system that's just not working for millions of people across this country. If I've gotten too hot I am sorry. That's what it takes. About who you are willing to stand up and own it."

Warren said she missed Senator Kamala Harris, who dropped out earlier this month. 

11:04 p.m. In a post-debate spin room interview with CBS News' Ed O'Keefe, Klobuchar said she's been waiting since the last debate to respond to Buttigieg's remarks about "100 years of experience" on the stage during that debate.

"I thought that was such an important factor about people's experience and we haven't talked about it enough on the stage," Klobuchar said. "And at the last debate, he said something about how 'Oh, the 100 years of experience on this stage, what has it given you?' And it never got answered because I didn't  have that chance. So I have been waiting for a month and I was able to do that."

Klobuchar continued that "I think it matters" how much experience a presidential candidate has before going to the White House.

"We've got a guy in the White House, who went in there with no political experience at all," Klobuchar said. "People are looking for someone to heal this nation and bring people together. So that factor, that you can get things done, I think it really matters."

Klobuchar is kicking off a 27-county Iowa tour on Friday, but she said she won't not let her campaigning get in the way of President Trump's Senate trial. She said she was recently in a "closed door" meeting with Senate Majority Leader Chuck Schumer, where they discussed the timing of the trial. 

"I'm not going to let this campaign get in the way of my duty and obligation as a juror in that very important proceeding," Klobuchar said. "And whatever, the chips will fall where they may. I think the people of this country will appreciate that." 

11:28 p.m.: Senator Cory Booker spent Thursday night phone banking in Des Moines after not making the criteria for the Democratic debate, CBS News campaign reporter Adam Brewster reported. He told a group of volunteers he believes he has a shot and "this ain't over until the people of Iowa speak."

Booker said he didn't think it was a "mortal blow" like it would be 3-4 months ago to not make the debate stage. But he admitted the exposure would have been an "incredible value" and it was "frustrating" to miss out on it.

"Every debate talked about as 'winner,"' he said. "CNN's focus group said we won it more than once. So it's exposure that is incredibly valuable. It always led to a spike in contributions. At the end of the last debate, the first hour after that was our best – it beat any day we had in the campaign. Anyone on stage with good performance benefits from that. We missed out on that, which is frustrating."

Booker said he's "confident" he'll make the debate stage in January. The Democratic National Committee hasn't released the qualifications for the January debate yet.

10:58 p.m.:   Biden imitated a stutter during the debate, in an attempt to show how children who stutter ask him for advice. Biden's struggle with his own stutter is well known, following a recent profile in The Atlantic. 

His words were swiftly mocked by former White House Press Secretary Sarah Huckabee Sanders, who tweeted "I I I I I I I I I I I I I I I hhhave absolutely no idea what Biden is talking about." 

After facing harsh blowback on Twitter, she clarified her comments, writing, "To be clear was not trying to make fun of anyone with a speech impediment. Simply pointing out I can't follow much of anything Biden is talking about." 

Biden also responded to Sanders' tweet, tweeting, "I've worked my whole life to overcome a stutter. And it's my great honor to mentor kids who have experienced the same. It's called empathy. Look it up." 

After Biden's response, Sanders deleted her original tweet and her first response. She then posted "I actually didn't know that about you and that is commendable. I apologize and should have made my point respectfully." 

10:38 p.m.: Moments after the debate wrapped up, Trump campaign press secretary Kayleigh McEnany's response to it landed in reporters' inboxes. 

"After yet another drab, pessimistic Democrat debate, it's even more clear why they felt they had to impeach President Trump. None of these characters has a chance," she said in a brief statement. 
 
The Trump campaign has been trying to make the case that every Democrat running in 2020 is extremely progressive, if not socialist, contrasting their policies with the economy under Mr. Trump.

It's unclear if the president, who was tweeting about Fox News coverage as of 10 p.m., watched any of the debate. 

10:25 p.m.: Candidates wrapped up the debate with brief closing statements, restating their cases for why they should be the president.

"I know what you're thinking America: How am I still on this stage with them?" Yang joked in his statement, before pivoting to his argument that politicians need to address the political symptoms that led to Mr. Trump's election.

Klobuchar finished a strong night by emphasizing her credentials as a moderate Democrat.

"If you are tired of the extreme in our politics, and the noise and nonsense, you have a home with me," she said.

Warren tacitly reiterated her earlier attack on Buttigieg for having big-money fundraisers, saying she would not work for the millionaires and billionaires. This theme was repeated by Sanders.

"The truth is, that real change always takes place from the bottom on up, never from the top on down," Sanders said.

Biden closed by arguing that he was the most electable candidate on stage.

"We all have big, progressive plans, and the question is, who can deliver on those plans," Biden said. He argued that he could help elect Democratic senators in red states, as well as defeat Mr. Trump.

10:18 p.m.: This question was met with some awkward moments of silence. Yang was forced to kick things off, as Warren joked that they could all boycott the question (all of the candidates had said they would not participate in tonight's debate if a labor dispute with Loyola Marymount University was not resolved beforehand). Yang said he doesn't think he needs to ask forgiveness from anyone. But he he offered his opponents his book as a gift, noting that Warren said she was already reading it.  

Warren said she speaks with so many voters, it's easy to grow weary in listening to each one. 

Biden said he and his wife keep lists of all the people they need to call on a weekly basis to talk through their problems. 

Sanders said he's written four books and would be happy to give them to anyone on stage. 

Klobuchar said she'd ask for forgiveness anytime it's needed, noting she can be blunt. But she suggested it's the president's behavior that can no longer be accepted. 

Steyer said the gift he'd like to give everyone on the stage is "the gift of teamwork." 

10:13 p.m.:  Moderator Tim Alberta asked Sanders if he would work on small-scale health care reform if Republicans retained the Senate, because a Republican Senate would be unlikely to support "Medicare for All." Sanders deflected, talking about the high costs that the U.S. currently spends on health care.

Biden called Sanders' plan unrealistic and touted his own plan, which would expand the Affordable Care Act passed during the Obama administration. He also took a jab at Sanders when the senator raised his hand while Biden was speaking.

"Put your hand for a second, okay, Bernie?" Biden said.

"Just waving to you, Joe," Sanders replied.

Biden slammed the high price of Medicare for All. Sanders acknowledged that taxes would go up in order to pay for it, but said that premiums and insurance costs would be eliminated.

Klobuchar also briefly clashed with Sanders.

"If you want to build a bridge over troubled water, you build a bridge, you do not blow one up," she said.

10:10 p.m.: Biden, who was vice president during the Obama-era troop surge in Afghanistan, was asked to respond to America's failures there and what he'd do next. The question comes days after the Washington Post published a scathing report about how U.S. officials over three administrations misled the American people about how poorly the war was going. 

Biden said he had opposed the surge in Afghanistan.

"Rebuilding that country as a whole nation is beyond our capacity," Biden said. 

The former vice president said he would send all combat troops home, negotiate with the Taliban, and keep some special forces on the ground. 

Sanders jumped into the debate. 

"You're also the guy that led us into the disastrous war in Iraq," Sanders said to Biden. "What we need to rethink is the entire war on terror." 

Buttigieg agreed that the troops need to come home, noting that when he left Afghanistan, years ago, he thought he was one of the last troops turning off the lights. 

10:08 p.m.:  Moderator Judy Woodruff asked candidates about the raft of federal judges who have been confirmed during Mr. Trump's time in office, and how their presence on the federal bench might stymie a Democratic president from implementing his or her policy priorities. However, Klobuchar and Buttigieg both deflected on the issue.

"We have to immediately start putting judges on the bench to fill vacancies so we can reverse the horrific effects of these Trump judges," Klobuchar said.

Senate Majority Leader Mitch McConnell blocked President Obama from filling several court vacancies, including a Supreme Court vacancy, in the final years of Mr. Obama's tenure, but has rushed to fill judicial vacancies under Mr. Trump. 

9:44 p.m.: Klobuchar and Buttigieg had one of the sharpest exchanges in the debate to this point, which came up during questioning about reparations for African-Americans who are descendants of slaves. 

Buttigieg said just crossing out a racist policy and replacing it with a neutral one isn't enough to reach equality. 

When it was Klobuchar's chance to address the matter, she suggested Buttigieg doesn't have enough experience for the job of president, reminding him he mocked the "100 years" of government experience on the stage during the last debate. 

"So while you can dismiss committee hearings, I think this experience works. And I have not denigrated your experience as a local official. I have been one. I think you should respect our experience," Klobuchar said. 

Buttigieg insisted that Klobuchar had denigrated his experience. 

Klobuchar hit on the Indiana's electoral records through the years, noting he lost in a statewide race.

"If you want to talk about the capacity to win, try putting together a coalition to bring you back to office with 80% of the vote as a gay dude in Mike Pence's Indiana," Buttigieg retorted, referring to his reelection as mayor in South Bend after he came out. However, South Bend is reliably Democratic. Since 1972, every one of its mayors has been a Democrat. 

9:40 p.m.:  Buttigieg said he supported providing financial remittance for people in families which were separated at the border.

Separately, he also talked about reparations for the descendants of slaves, saying that he supports H.R. 40, a bill to study the implementation of reparations. He also talked about the need to invest in minority-owned businesses.

Biden demurred on the issue of reparations for the descendants of slaves, but instead focused on the importance of immigration in the U.S.

9:33 p.m.: Asked about immigration, Yang first pivoted back to the lack of dynamic leadership in the U.S., saying the nation is "deeply misogynist, and most of us know that." 

The nation needs more women in leadership, and strong men don't have any problem with that, he said. 

"If you get too many men alone and leave us alone for too long, we kinda become morons," Yang said, prompting laughs from the audience. 

9:21 p.m.:  Warren went after Buttigieg for holding high-dollar fundraisers, including one in a "wine cave" that was $900 per person last week.

"Billionaires in wine caves should not pick the next president of the United States," Warren said.

Buttigieg noted that he was "literally" the only person on the stage who wasn't a millionaire or a billionaire.

"This is the problem with issuing purity tests you yourself cannot pass," Buttigieg said.

"I do not sell access to my time," Warren replied.

Buttigieg rejoined that Warren had transferred some of her money from her Senate fundraising account, which had raised some money from large donors, to her presidential campaign.

Klobuchar interjected that candidates needed to focus on issues like campaign finance reform, eliciting some of the largest applause of the night.

"I did not come here to listen to this argument," Klobuchar said. "And I've never even been to a wine cave." She quipped that she has been to a wind cave, though, in South Dakota.  

Sanders also attacked Biden and Buttigieg for having billionaires donate to their campaigns. 

9:15 p.m.: Moderator Tim Alberta pointed back to former President Obama's recent comments that one of the nation's greatest obstacles are "old men" who won't get out of the way. 

Sanders, a 78-year-old white man, said he has great respect for Mr. Obama, but he said on this, he disagrees. 

"The issue is not old or young, male or female, the issue is working people standing up, taking on the billionaire class, and creating a government, an economy, that works for all, not just the 1%," Sanders said. 

Biden stated he didn't think the former president was referring to him. Mr. Obama's former vice president also addressed reporting that he might not run for a second term if he wins the election. 

"No, I'm not willing to commit one way or another" to running for a second term," Biden said.  "Let's see where we are, Let's see what happens."

When Alberta pointed out to Warren that she would be the oldest president ever inaugurated, she had a quick retort. 

"I'd also be the youngest woman ever inaugurated," Warren retorted. 

9:23 p.m.:  In the first hour, Klobuchar spoke the most, speaking 8 minutes and 32 seconds with five questions and two rebuttals. She was followed by Buttigieg, who had 7 minutes and 39 seconds after five questions and three rebuttals.

Here is the full breakdown of the candidates and their speaking times:

• Klobuchar: 8:32 (5 questions, 2 rebuttals)
• Buttigieg: 7:39 (5 questions, 3 rebuttals)
• Warren: 7:09 (5 questions)
• Joe Biden 6:53 (6 questions)
• Bernie Sanders 6:35 (5 questions, 1 rebuttal)
• Tom Steyer 6:05 (3 questions, 2 rebuttals)
• Andrew Yang 5:24 (4 question, 1 rebuttal)

9:17 p.m.: Moderator Amna Nawaz asked Yang what message is being sent when he's the only non-white candidate on the debate stage, and the Democratic field overall is "overwhelmingly white."

"It's both an honor and disappointment to be the lone candidate of color on this debate stage tonight," Yang said.

He said he missed Senator Kamala Harris, who ended her campaign this month despite qualifying for the debate, and Senator Cory Booker, who did not qualify. "Though I think Cory will be back," Yang added, with a snap of his fingers.

Yang mentioned that he is the son of immigrants and faced racism growing up. But he acknowledged that black and Latino Americans face steeper economic hurdles, making it harder for them to get far in the political process.

Yang then plugged his freedom dividend — his campaign pledge to give every American $1,000 a month. He said he could "guarantee" that this would have paved the way for more candidates of color.

"You know what you need to donate to political campaigns?" Yang asked. "Disposable income."

9:02 p.m.:  Sanders, who is Jewish, said that it is important to support Israel, but that American foreign policy should also be focused on the plight of Palestinians.

"We must be pro-Palestinian as well," Sanders said.

Sanders also slammed Israeli Prime Minister Benjamin Netanyahu, who has been a strong ally of Mr. Trump. Citing the expansion of Israeli settlements during Netanyahu's tenure, and pointing to his legal problems, Sanders said, "we must understand that right now in Israel we have leadership under Netanyahu who has recently as you know been indicted for bribery, who, in my view, is a racist."

Biden also condemned Netanyahu, although in softer terms.

"Bibi Netanyahu and I know each other well. He knows that I think what he's doing is outrageous," Biden said.

8:53 p.m.: The debate pivoted to questions about racial tensions and racial diversity in America. 

Sanders, asked how he'd address racial inequality, began to talk about climate change.

"First, I want to go back to climate change," Sanders said, attempting to address the issue further.

"With all due respect, Senator Sanders, the question was about race," PBS journalist Amna Nawaz interjected, steering the Vermont senator back to the question at hand. 

Only one of the candidates on stage Thursday night, Yang, is not white, a lack of diversity that has been criticized. 

Sanders went on to weave the issues of race and climate change together, insisting climate change is a vital issue for communities of color. 

8:50 p.m.:  Moderator Amna Nawaz asked Biden about his desire to work with Republicans, and his sense that politics would "return to normal" after Mr. Trump leaves office. 

"I didn't say, 'return to normal.' Normal's not enough," Biden replied. "With Trump out of the way, it's not going to change things in a fundamental way," he said, except that it would mean Mr. Trump's base would no longer be able to "intimidate" centrist senators.

He also obliquely referenced recent attacks by Republicans and especially by Mr. Trump.

"The way they've attacked me, my son, and my family — I have no love," Biden said. Mr. Trump asked the Ukrainian president to investigate Biden and his son, Hunter, in a July 25 phone call that was central to whistleblower complaint that kicked off the impeachment inquiry. 

8:44 p.m.: Asked if he'd be willing to see thousands of blue collar workers in the oil and gas industries lose jobs and relocate them to improve the climate, Biden said yes. 

"We have to make sure we explain it to the people that are displaced" that their skills will be used elsewhere in the new economy, Biden said.

Warren said the greatest obstacle to addressing climate change is the lobbyists, lawyers and think tanks in Washington who are failing to step up to the plate and address the problem. As she often does, Warren directed the climate change question back to the theme of "corruption." 

Klobuchar also tied climate change and corruption. 

"The way we tackle this corruption is by winning big in this election," Klobuchar said. 

Yang said the country needs to "have nuclear on the table" in order to address climate change. 

Steyer jumped in, saying nuclear power isn't where it needs to be yet in the U.S. 

"We actually have the technology that we need. It's called wind and solar and batteries," Steyer said. 

8:40 p.m.:  Moderator Tim Alberta asked candidates whether they would support a federal program to relocate citizens of cities which will be hard-hit by climate change, such as Miami, Florida. Alberta noted that scientists have predicted low-lying cities in particular will definitely be affected.

Candidates deflected the question, instead pivoting to their own plans on climate change.

"I very much hope we're not going to have to relocate entire cities, but we're probably going to have to relocate certain individuals," Klobuchar conceded.

Buttigieg, the youngest candidate on stage, said that he lived in a city on a river, "so I know what's at stake."

"We have to summon the energy of the entire countries to deal with this," Buttigieg said.

Yang was the only candidate to address the question.

"We should obviously be paying to relocate Americans that are hit by climate change," Yang said. 

8:24 p.m.: Warren was asked how she responds to economists who criticize her proposed tax on the wealthy as stifling economic growth.

Warren said those economists are "just wrong."

Billionaires, Warren said, aren't investing back in the economy, or buying "pizzas." That money can be better used for things like child care, she said.

Tom Steyer, a billionaire, said he's been in favor of a wealth tax "for over a year." 

Buttigieg said taxes on the wealthy and corporations need to go up, but that needs to be done wisely. He called it a "false choice -- you either have to go all the way to the extreme or it's business as usual."

"We've got to break out of the Washington mindset that measures bigness of an idea by how many trillions we spend or the boldness of an idea by how many fellow Americans it can antagonize," he argued. 

8:16 p.m.:  Impeachment was not the only hot topic from Congress that was addressed on the debate stage. The moderators also asked the candidates about the U.S.-Mexico-Canada trade agreement (USMCA), which was approved in the House on Thursday.

Sanders, a longtime skeptic of trade deals, said that he did not believe the USMCA went far enough to protect workers or address climate change. Sanders noted that he voted against NAFTA and called the USMCA "a modest improvement over what we have right now."

"No, I will not be voting for this agreement, though he conceded "it makes some modest improvements."

However, Klobuchar countered that she would support the USMCA, noting that Senator Sherrod Brown of Ohio — a strong workers' rights advocate — was going to support the deal.

"I think this agreement, while Senator Sanders is correct there are some issues with it, is much better than the one originally proposed," Klobuchar said, saying that House Democrats had negotiated worker-friendly provisions in the deal.

8:07 p.m.: The first question went to Biden, who was asked what he'd do to persuade the roughly half of Americans who don't think President Trump should have been impeached. 

"We need to restore the integrity of the presidency," Biden said, adding it's his job to make sure Mr. Trump isn't in office another four years. 

Sanders, given the same question, claimed Mr. Trump has sold out the working people of this country, and has lied "thousands" of times since taking office. 

"We have a president who is a pathological liar, we have a president who is running the most corrupt administration in the history of this country," Sanders said. 

Warren accused the president of normalizing corruption, even after he promised to "drain the swamp." 

"From tax breaks to ambassadorships, we have to prosecute the case against him and that means" the country needs a candidate who can draw the sharpest distinction, Warren said. 

"That's why I'm in this race," she added. 

Klobuchar said what the president did in attempting to get a foreign nation to intervene in the U.S. election is exactly why the founders created the tool of impeachment. 

"This is a global 'Watergate,'" Klobuchar said. 

Klobuchar called for the president's top aides, like acting White House chief of staff Mick Mulvaney, to testify. 

"If President Trump thinks that he should not be impeached, he should be not scared to put forward his own witnesses," Klobuchar said. 

Buttigieg said that no matter what happens in any Senate trial, Americans have the chance to vote in 2020. 

"We cannot give into that sense of helplessness, because that's what they want," Buttigieg said. 

Yang said American leaders aren't focusing on the problems that led to Mr. Trump's election, largely blaming the media for getting the country to this point. 

"We have to stop being obsessed with impeachment … and start actually digging in and solving the problems that got Donald Trump elected in the first place," Yang said. 

7:51 p.m.:  The Yang campaign announced shortly before the debate that actor and rapper Donald Glover will be joining the campaign as a creative consultant. Glover and Yang appeared at a pop-up event in Los Angeles earlier Thursday. The first people who arrived got limited edition hoodies -- only 10 were available, for $1,000 each.

In a brief gaggle with reporters, Yang said his team was approached by Glover about a collaboration between the two, and that Yang said he was excited to work with a creative mind he really admires, CBS News campaign embed Ben Mitchell reported from the event.

7 p.m.:  Health care is the top concern for Democratic voters, and Elizabeth Warren has been facing scrutiny appearing to back off her Medicare for All plan. She's still ultimately seeking a government-run plan, but has been spending more time talking about what it would take to make that happen, telling voters they'd continue to have a "choice" during a transition phase. That word choice has led to criticism from liberal and moderate opponents who say she's wavering on the subject. But Warren disputes that. 

Former New York City Mayor Michael Bloomberg, who has made a bit of a splash so far, won't be on the stage tonight -- but is making his presence felt.

He's spent millions of dollars on advertising nationwide so far and on Thursday, he took a swipe at the field for lacking something he has -- management experience.  The former mayor singled out Joe Biden, saying he's never managed an organization or run a school system. Biden ignored questions about this at a stop here in L.A. on Thursday, but if asked, he'd likely remind the mayor he was once just a heartbeat away from the presidency.   

Will there be a candidate who emerges as the frontrunner to beat, and therefore the most popular target for attack on the debate stage? 

Elizabeth Warren has been that candidate in the past, but she's been fading in the polls lately. Whether the candidates treat Bernie Sanders, Joe Biden, Pete Buttigieg or someone else as the frontrunner will speak volumes. 

Delivering a strong performance will be important for Biden, whose off-the-cuff comments and gaffes have unsettled some in his party. 

The candidates could be asked about the lack of racial diversity on stage, too. Andrew Yang will be the only candidate of color at the debate. Many Democrats have lamented the whiteness of the top tier.

Several of the Democrats running for president are U.S. senators, meaning impeachment will keep them busy in January.

That's when the Senate is expected to hold a trial to decide whether to remove President Trump from office. It's an unlikely outcome since the Senate is dominated by Republicans and a two-thirds vote is required.

Still, Senators Cory Booker, Amy Klobuchar, Bernie Sanders and Elizabeth Warren have to attend the trial, which would take place six days a week until it's over. The trial of Bill Clinton, the last president to be impeached, lasted over a month. 

Senate Majority Leader Mitch McConnell has expressed his desire for a quick trial, and unlike Clinton's, without witnesses. But that would still take the senators running for president off the campaign trail for a little while at a crucial time. The Iowa Caucuses, which kick off the primary season, take place on February 3. 

"We will do our best to get back to Iowa, to get to New Hampshire, to get to all of the states that we have to, but there's no question it will make our life a little bit more difficult," said Sanders.

On the ground, it'll mean that top-tier candidates Pete Buttigieg and Joe Biden will have the early states to themselves for part of the month.

President Trump, now the third U.S. president to be impeached, isn't expected to be removed from office, and the candidates vying to replace him have largely avoided bringing up impeachment on the campaign trial. But almost all of them have weighed in since he was impeached.

Elizabeth Warren used the opportunity to fundraise.

"It's important to remember that Trump is just the worst symptom — not the cause — of a rigged, corrupt system. A system that rewards the rich and powerful and leaves working people behind. 2020 is our chance to change that," an email to her supporters read.

Andrew Yang also pivoted from impeachment to his platform.

"Watching impeachment unfold is like watching a game when you know the final score," he tweeted. "If the media spent a fraction of the time they are spending on impeachment on the economic dislocations that got Trump elected we would be a stronger country for it."

Others expressed sadness, including Cory Booker, Bernie Sanders and Joe Biden, who is a central figure in the impeachment.

"This is a solemn moment for our country. But in the United States of America, no one is above the law — not even the President," he tweeted.

Read more.

These are the seven candidates who qualified for the debate: 

• Former Vice President Joe Biden

• Mayor Pete Buttigieg

• Senator Amy Klobuchar

• Tom Steyer

• Senator Bernie Sanders

• Senator Elizabeth Warren

• Andrew Yang

Candidates were required to meet higher thresholds in the sixth debate, compared to prior debates. They had to prove that they have at least 200,000 unique donors, including at least 800 in 20 different states, U.S. territories or the District of Columbia. That's an increase from 165,000 last month. Contenders also had to reach either 4% in at least four national or early-state polls or reach 6% in two early-state polls. That's an increase from 3% and 5%, respectively.