With control of the Senate at stake, these are the races to watch

There are 36 Senate races to be decided on Election Day but only one-third of them are considered close enough for Republicans and Democrats to seriously worry about. And it's those races that will determine whether Republicans or Democrats control the Senate come January.

Currently, Democrats hold 55 seats (including two independents who caucus with the Democrats), the Republicans have 45 seats. Picking up six seats on November 4 would give Republicans a majority, and they'll have plenty of opportunities.

In three states currently held by Democrats - Montana, South Dakota and West Virginia - Republican victories are all but certain, leaving the GOP only three pickups to wrestle control from the Democrats.

In North Carolina, New Hampshire, Arkansas, Colorado, Alaska and Louisiana, Republican challengers threaten to knock off Democratic incumbents. Three more races - Iowa, Georgia and Michigan - feature open races where the Republican candidate stands a good chance, offering nine possibilities in total.

It's not all about the Republicans on the offensive, however. While far fewer than the Democrats, Republicans still have a few seats they'll have to vigorously defend: Kentucky, Georgia and, to a lesser extent, Kansas.

But the numbers aren't the only thing working in the GOP's favor; Democrats are also tied to an increasingly unpopular President Obama, who has an approval rating hovering around 40 percent nationally and even lower in many of the states Democrats are defending. That means Mr. Obama probably won't spend much time campaigning, said CBS News Political Director John Dickerson.

"What's motivating Republicans in this election is disappointment with the president," Dickerson said. "If you ask Republicans, more of them strongly disapprove of the job he's doing than in 2010."

It's not all bad news, though, he said.

"Democrats do benefit from the president. He can raise money, and he can rally the base of the Democratic Party and that's important in a non-presidential election year when Democrats don't turn out to vote the way Republicans do," Dickerson said.

Instead of focusing on the president or any of the foreign policy crises affecting the country, Democrats are trying to keep their races tightly focused on local issues or their opponent's flaws, he added.

"Anything that focuses on the unpopular president might rally Republican voters, or at the very least distract from a Democratic candidate's local message," he said.

Here are the 11 Senate races to watch between now and the midterm elections:

Kentucky

Sen. Mitch McConnell (R) vs. Alison Lundergan Grimes (D)

Democrats would like nothing more this fall than to take down Senate Minority Leader McConnell, the Democrats' chief antagonist in the Senate. Ever since he said in 2010 that the chief Republican goal was to make Mr. Obama a one-term president, Democrats have painted him as the chief obstructionist.

McConnell, running for his sixth term, is facing Kentucky Secretary of State Grimes, who has attacked McConnell for his low approval rating in the state.

Both candidates have hit stumbling blocks this summer: Lundergan Grimes appeared unfamiliar with Israel's Iron Dome system at the height of the Gaza crisis, and later faced scrutiny over her campaign's use of a charter bus from her father's company.

McConnell, for his part, was secretly recorded earlier this summer explaining his plans to aggressively roll back the president's agenda across the entire federal government if Republicans take over the Senate. And just last week, his campaign manager, Jesse Benton, resigned amid fallout from a scandal in Iowa connected to a presidential campaign he worked for.

While a handful of polls throughout the first six months of the year put Grimes ahead, McConnell appears to have edged out a small lead in the last two months. A CBS News/New York Times estimate from July put him up by four points, 50 percent to 46 percent, and a Bluegrass Poll from late August has him holding a 46 to 42 percent lead among likely voters. Grimes is still within the margin of error, however.

North Carolina

Sen. Kay Hagan (D) vs. Thom Tillis (R)

Hagan was elected in 2008 when Mr. Obama's election helped boost Democratic turnout statewide, but is vulnerable in large part because of her support for the Affordable Care Act, which is very unpopular there.

She faces North Carolina House Speaker Thom Tillis, an establishment-backed Republican who fended off a tea party challenge in the primary but has also presided over a rightward shift in the state's politics. The state legislature has grown increasingly unpopular under his leadership, a fact Hagan has sought to exploit, pointing to policies and budget cuts she says have damaged the state's public education system.

But Tillis doesn't miss an opportunity to remind voters that Hagan frequently aligns with the president, and has sought to tie her to his policies on immigration and foreign policy. Hagan has said Mr. Obama should not take executive action on immigration, which he has been promising, and suggested that the rise in Islamic militants in Iraq and Syria occurred because the administration failed to arm the moderate Syrian rebels soon enough.

She has also accused Tillis of having a terrible record on women's issues, which he sought to dispute by coming out in favor of increased access for over-the-counter oral contraceptives during the first debate in early September. He was just the fourth GOP candidate to do so.

A CBS News/New York Times estimate put Tillis up over Hagan by just one percentage point in July, and a USA Today/Suffolk poll from mid-August put Hagan up by two points.

Arkansas

Sen. Mark Pryor (D) vs. Tom Cotton (R)

Pryor is the lone Democrat in Arkansas' congressional delegation and one of the most vulnerable incumbents facing re-election. He has a lineage in the state's politics: his father, David Pryor, was a governor and senator. He is one of the most moderate Democrats in the chamber, though he is hampered by his vote for and continued support of the Affordable Care Act (though he says it needs improvements) and Arkansas' lurch to the right.

His bid for a third term is being challenged by Rep. Tom Cotton, who was one of the GOP success stories of the 2012 election and a rising star in the party. Cotton holds two degrees from Harvard and is a veteran of both the Iraq and Afghanistan wars, and has tea party support to boot. He has generous financial backing from outside conservative groups, although Pryor still has an overall edge in money.

The tight race has led to some desperate moves by Pryor, including a campaign ad last week that linked the Ebola outbreak to a vote Cotton took to slash funds for certain emergency medical programs.

Cotton has had a slight edge in most recent polls, including a four-point lead in the July CBS News/New York Times estimate. But his leads are often limited to one or two points and stay within the margin of error.

Iowa

Bruce Braley (D) vs. Joni Ernst (R)

Rep. Bruce Braley, D-Iowa, and Republican state Sen. Joni Ernst are locked in perhaps the closest election battle in the nation as they compete to fill the seat vacated by retiring Sen. Tom Harkin, D-Iowa. Several recent polls show them tied, and the CBS News/New York Times estimate in July had Ernst up by just one percentage point. A Suffolk Poll released in late August put Braley ahead - by just .02 percent.

Ernst drew notice amongst a crowded Republican primary field for a campaign ad in which she said she would be able to draw upon her childhood experience working on a farm castrating hogs when she went to Congress.

"Washington's full of big spenders," she says at the end of the ad, which won approval from former vice presidential candidate Sarah Palin. "Let's make 'em squeal."

Braley, for his part, faced blowback after making remarks at a closed-press event in which he suggested Iowa Republican Sen. Chuck Grassley was unfit to become the Senate Judiciary Committee chairman (a post he would hold if the GOP controls the Senate) because he is a "farmer from Iowa who never went to law school." The remarks were caught on tape and Braley later apologized.

But it's not the only issue that opponents have raised to question his character. There was an incident where a neighbor at the Braley's vacation property in Iowa said the congressman threatened to sue when her chickens kept wandering into his yard, which Braley denies. He was filmed telling a few Iowans at a July 4 parade that he, too, was a farmer, which his campaign said was a result of him mishearing what the people originally said to him. And opponents said an ad he ran against Ernst was sexist because it compared her to a chick.

Georgia

Michelle Nunn (D) vs. David Perdue (R)

Seeking to replace retiring Sen. Saxby Chambliss, R-Ga., Michelle Nunn, the daughter of former Sen. Sam Nunn, had little trouble securing the Democratic nomination and was able to avoid an immediate challenger for a while due to the Republican runoff between Rep. Jack Kingston, the establishment pick, and David Perdue, a businessman and former CEO of both Reebok and Dollar General.

Though he was not the establishment favorite, Perdue prevailed. Both he and Nunn are running as political outsiders - Nunn was the CEO of volunteer organization Points of Light, which was founded by former President George H.W. Bush - meaning they will compete for moderates and independents, even though the state leans Republican.

The CBS News/New York Times estimate in July showed Perdue leading by 6 percentage points, 50 to 44, though local station WSB-TV in Atlanta put Nunn ahead, 47 percent to 40 percent, in late August. She had a nearly 20-point lead among women in that poll, as well as an edge among black voters.

Louisiana

Sen. Mary Landrieu (D) vs. Bill Cassidy (R)/Rob Maness (R)

Landrieu is another one of the Democrats fighting for her political life in a state where President Obama is relatively unpopular. And it may not be over on Nov. 4: in Louisiana's jungle primary system, all candidates appear on the same ballot, regardless of party, and the winner must receive at least 50 percent of the vote. If no one does that on Election Day, there will be a runoff on Dec. 6. That could also leave the question of Senate control open until then.

The two strongest challengers to Landrieu, who is hurt by Louisiana's increasing lean to the right, are Cassidy, who serves in the House of Representatives, and Maness, a retired Air Force colonel. A July CBS News/New York Times estimate between Landrieu and Cassidy put Landrieu at 46 percent and Cassidy at 47 percent. There has been very little polling pitting the three top candidates against each other.

The latest twist for Landrieu is a question of her residency, which has become an issue for some Senate candidates this year. A Washington Post report in late August revealed that the three-term senator claims her parents' home address in New Orleans as her voting residence, and her campaign says that's because she lived there for most of her life and stays there when not in Washington or traveling around Louisiana. But Landrieu and her husband also own a $2.5 million house just four blocks from the U.S. Capitol in Washington.

Alaska

Sen. Mark Begich (D) vs. Dan Sullivan (R)

Though Begich, the former mayor of Anchorage, managed to oust six-term Sen. Ted Stevens (who had received a felony conviction) in 2008, he's in trouble in 2014. Alaska has always been tough territory for Democrats, and former state Attorney General Dan Sullivan is looking to restore a Republican to the seat.

Begich has worked hard to distance himself from President Obama, avoiding talk of the president's health care law whenever possible and promoting differences with the president like oil and gas development and gun rights. He's also run hard against Sullivan, who had to survive a tough Republican primary, but he may have run too hard: on Tuesday, Begich's campaign was forced to pull a controversial ad that blamed Sullivan for clearing the early release of a convicted sex offender who is now accused of murdering an elderly couple and sexually assaulting their 2-year-old granddaughter. Sullivan's campaign condemned the spot, and an attorney for the victim's family asked both campaigns to take the ads down.

Begich has led in most polls, including a 12-point lead in the CBS News/New York Times estimate. But Alaska polling can be inaccurate, and with results coming in late on Election Night due to the time difference, this race could be the deciding factor in control of the Senate.

Colorado

Sen. Mark Udall (D) vs. Cory Gardner (R)

Gardner is one of the rising stars of the Republican Party and was the decisive choice of Republican delegates to face Udall in the general election. And though Udall won his first race by 10 points, the state seems to be swinging toward the right in the last few years.

Facing a strong challenge from Gardner, Udall has looked to create distance from the president. Though Mr. Obama traveled to Denver to benefit Udall's campaign and other Senate Democrats, Udall skipped the event at the last minute, saying he had to attend to business in Washington.

There has been little recent polling in the race, but the July CBS News/New York Times estimate put Udall up 51 percent to 47 percent over Gardner. But a Quinnipiac Poll around the same time had Gardner up by two percentage points. Udall has raised $13.5 million to Gardner's $5 million, but has also spent nearly $8 million of that, compared to Gardner, who has spent just $2 million.

Michigan

Gary Peters (D) vs. Terri Lynn Land (R)

Rep. Peters continues to lead Land, a former Michigan secretary of state, in recent polls of the race to replace retiring Sen. Carl Levin, D-Mich. But his lead continues to grow smaller, with Land gaining three percentage points between two surveys conducted by EPIC-MRA between July and August. Their August survey has him up by 6 percentage points.

Neither candidate faced a primary challenge, leaving them to focus on each other the entire time. Land has questioned Peters' commitment to border security as part of immigration reform, and Peters has been questioning millions of Land's contributions. Land has a slight edge in both fundraising and cash on hand, and the Detroit Free Press estimates that nearly $13 million in outside money has poured into the race.

New Hampshire

Sen. Jeanne Shaheen (D) vs. Scott Brown (R)

Former Massachusetts Sen. Scott Brown is looking for a return ticket to Washington - only this time, he wants to go as the senator from New Hampshire. After winning the 2010 special election to replace the late Sen. Ted Kennedy, and then losing the seat to Sen. Elizabeth Warren, D-Mass., in 2012, Brown moved to New Hampshire looking for redemption.

He entered the race running hard against Obamacare, and until a few days ago it looked like a long shot. Shaheen, a former governor of the state who was elected to the Senate in 2008, has led Brown in every single poll of the race. But a WMUR-TV/University of New Hampshire poll released in late August showed Shaheen with a mere 2-percentage point lead, within the margin of error of the poll. That same poll had her up 12 points just a month earlier, and the CBS News/New York Times estimate in July had her leading by 10 percentage points, 52 percent to 42 percent.

Brown embraces opportunities to tie Shaheen to Mr. Obama and point out problems in the world, ranging from the advance of Islamic militants in Iraq to the flood of Central American children at the southern border. Shaheen, on the other hand, has talked more about pocketbook issues that affect families, like the cost of child care, student loans and infrastructure.

Kansas

Sen. Pat Roberts (R) vs. Greg Orman (I)

Roberts' biggest impediment to re-election seemed to come in the form of a Republican primary challenge from local radiologist Milton Wolf and questions over his residence (he rents space from supporters in the house he bills as his official residence in Kansas). Still, he beat Wolf by 48 percent to 41 percent in the August 5 primary and looked like he had smooth sailing ahead.

Not so fast. New polls are suggesting that businessman Greg Orman, a Democrat-turned-independent with plenty of money to help fund a challenge, could be a problem for Roberts. Recent Democratic polling showed that in a head-to-head matchup, Orman would beat Roberts.

There's a twist to that too: the survey also showed that Orman only wins if Democrat Chad Taylor, a district attorney, isn't in the race.

Seemingly recognizing that fact, Taylor moved to withdraw his name from the ballot on Sept. 3 but was told by the Kansas Secretary of State, Republican Kris Kobach, that he had not used the necessary language. Kobach said Taylor's name will remain on the ballot, which could help Roberts, and Taylor said he plans to challenge the decision.

Roberts' campaign, meanwhile, has alleged Taylor's move is part of a "corrupt bargain" between Orman and Democrats.

Even though Orman used to be a Democrat, he has been very critical of both parties in Washington, and only says he'd likely support the majority party if he won. He's being coy about what would happen if he decided which party had the majority.

"If I get elected, there's a reasonable chance neither party has a majority in the U.S. Senate," he told the Washington Post. "And if that's the case, what I would do is sit down with both parties and have a real frank discussion about the agenda they want to follow."

But Taylor hasn't revealed any plans to leave the race, and though he told the Democratic Senatorial Campaign Committee (DSCC) not to help him, Taylor's campaign told the Post they have also not encouraged him to drop out.