But he rejects the idea that Clinton's campaign -- which many believe has been on a hot streak lately following an appearance on "Saturday Night Live," a well-received debate performance and Vice President Joe Biden's decision to not seek the White House -- ended the week on a high note following her lengthy testimony before the House Select Committee on Benghazi.
"People may think she had a good week," Rubio told CNN's Jamie Gangel in an interview that aired Sunday on "State of the Union." "I think this is the week it was proven that she lied about Benghazi."
The Florida senator said he believes Clinton will be the Democratic nominee, thanks in part to her belonging to a "political dynasty" that will help her fundraising.
As for Donald Trump, the party's current front-runner, Rubio charged the developer's immigration proposals "border on the absurd."
"His rhetoric is a little louder but, if you think about where he was six months ago, his position on immigration six months ago was nothing like what he's saying now. And even what he's saying now borders on the absurd," he said. "He's going to deport all these people and then he's going to allow back in the ones that are good. His plan makes no sense."
Overall, Trump has failed to communicate a clear, deep understanding of America's most pressing issues, Rubio said.
"I would say that ultimately the next president of the United States, on their first day in office, must understand the threats that face this country and must have shown good judgment about what to do about those issues," he said.
And despite Trump's current success, Rubio is confident that the former reality television star's popularity will wane.
"I'm going to support the Republican nominee, and I'm comfortable that it's not going to be Donald Trump, and I'm increasingly confident that it's going to be me," he said.
Confidence and optimism aside, Rubio isn't blind to his low presidential poll numbers -- even in his home state -- and blames them on "a very unusual year."
"If you start paying attention to these polls in October, I mean, you'll go crazy. I've been up, I've been down," he said. "It's a very unusual year, and I think part of it is that people are really angry about the direction of our country."