Sen. Marco Rubio on terror threats, 2016 elections and new book

As terror threats against western countries rose, world leaders gathered in Paris Sunday to rally for unity, denouncing the three days of attacks against France that took the lives of 17 people and left three gunmen dead.

Notably missing from a rally that included several European heads of state, as well as Israeli President Benjamin Netanyahu and Palestinian Authority President Mahmoud Abbas, was President Obama, or any top U.S. official.

"I thought it was a mistake not to send someone," Florida Sen. Marco Rubio said Monday on "CBS This Morning. "Look, I understand that when the president travels, he brings with him a security and communications package, which is intense, and I understand you drop that into the middle of something like this, it could be disruptive. But Eric Holder was in Paris, and maybe John Kerry should have gone, or somebody else. There's a plethora of people they could have sent."

Rubio, who is a member of the Senate Intelligence and Foreign Relations Committees, said people will take cues from the absence of a senior U.S. official, especially since nations around the world rallied around America when the country was suffering greatly from 9/11.

"The French are going through a similar trauma," Rubio said.

The White House admitted later Monday that it erred in not sending a higher-profile member of the administration to the rally.

In terms of attacks targeting the U.S., Rubio said he's more concerned about the "new wave" of terrorism, in which individuals or small groups of people carry out smaller-scale attacks.

"We've always known that that has been an aspirational goal of multiple jihadist groups around the world, now we're actually starting to see it carried out," Rubio said.

Rubio said while U.S. officials work to identify the individuals who pose threats, they're not always going to succeed.

"No one can promise you that one wouldn't slip through the cracks. And part of the reasons why I feel so strongly against these efforts to diminish America's intelligence-gathering capabilities is because of the threat that this poses," Rubio said. "In fact it is easier to disrupt the plot of 25 people than it is the plot of one or two or three."

Furthermore, Rubio said terrorists adjust what they do when journalists report on national security disclosures.

"[The terrorists] learn from it and they say we're going to stop doing that. ...We'll use a different carrier or a different method of communication. And so we have to constantly adjust to that, and it's a real challenge," Rubio said. "No one should understate what a big, big challenge this is moving forward."

Rubio has been considering whether he runs for president or seeks re-election as senator, but he has yet to announce his decision.

Part of his decision-making process is wrapped up in the publication of his new book, out Tuesday, "American Dreams: Restoring Economic Opportunity for Everyone." It outlines his argument for why it's becoming increasingly difficult for people, particularly the middle class, to achieve the American dream.

"Where's the best place for me to achieve the agenda that I outline in 'American Dreams?' Is it the Senate, in the majority now, or is it as president of the United States?" Rubio said.

With increased global competition, demands for skills and education, and rising costs, Rubio said millions of people in the U.S. "feel that the American dream is slipping out of reach."

"As a nation, that's problematic because it undermines our identity as a nation of equality, of opportunity and upward mobility," he said. "If we don't address it, we'll lose what makes us special."

Although official declarations could still be weeks or months away, there are signs the Republican field will be crowded. Both Jeb Bush and Mitt Romney have expressed interest in running, and Rubio acknowledged they could be "formidable candidates," both very capable of raising significant campaign funds. However, if Rubio concludes that the best way for him to serve the U.S. would be as president, he said he's confident he'll be able to put a campaign together that makes him competitive.

"If I run for president, it won't be against anyone," Rubio said. "It'll be because I believe I have an agenda that no one else is offering on our side of the aisle, and I believe I can do a better job than they would."

The more candidates, the better, he said, "because it gives us a chance to debate these ideas."

Rubio has also been vocal on foreign policy. A Cuban-American, he has criticized the president for beginning to normalize relations with Cuba. Even though Cuba announced Monday it had released 53 prisoners as part of the deal, Rubio was still skeptical that Cuba would change its ways.

"I don't mean to diminish it for those 53, but in return for that [release], the Cubans are getting virtually everything they want from the Obama administration," Rubio said.

In the last two years alone, Rubio said, the Cuban government has arrested thousands of people, so the 53 people released represent "minimal changes."

"I don't have a problem with changing U.S. policy towards Cuba, but I think it has to be a reciprocal change, where economic openings equals reciprocal political openings on the island," Rubio said.

Rubio stressed that his interest in Cuba is to promote political reform, adding that there is no contemporary example of a country in which greater prosperity led to change in autocratic regimes. He used China, Vietnam and Burma as examples.

"So my belief is that the Cuban government will pocket all of these changes to benefit the regime, which completely controls the economy, but it has already been made very clear - there will be no political changes on the island of Cuba," Rubio said.