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Could Sen. John McCain, R-Ariz., really cast a ballot for Democrat Hillary Clinton if she faces Kentucky Sen. Rand Paul in a 2016 presidential matchup? It would be a "tough choice," the Arizona senator joked in an interview with the New Republic.

"When Hillary Clinton versus Rand Paul occurs in 2016, I guess you are going to have to decide who to vote for, huh?" the New Republic asked McCain.

"It's gonna be a tough choice," McCain said, laughing.

McCain, a former presidential candidate himself, then clarified his views on tea party-favorite Paul, whose father, former Rep. Ron Paul, R-Texas, previously made a bid for the White House.

"I think that Rand Paul represents a segment of the GOP, just like his father. And I think he is trying to expand that, intelligently, to make it larger," McCain said.

McCain and Paul have often found themselves on opposing sides when it comes to national security and foreign policy issues. Earlier this year, McCain even went as far as calling Paul, Sen. Ted Cruz, R-Texas, and Rep. Justin Amash, R-Mich., "wacko birds," for which he later apologized.

"They were elected, nobody believes that there was a corrupt election, anything else," McCain said then in an interview with the Huffington Post. "But I also think that when, you know, it's always the wacko birds on the right and left that get the media megaphone."

McCain and Clinton served together in the Senate and have had a cordial relationship, but McCain recently became critical of Clinton's involvement in the administration's handling of the attacks in Benghazi, Libya, last year. As for Clinton's role as secretary of state, McCain said "she did a fine job."

"She's a rock star. She has, maybe not glamor, but certainly the aura of someone widely regarded throughout the world," McCain told the New Republic. "I do think it is interesting that the issues where John Kerry is engaging is where Hillary Clinton did not engage in, that those decisions were left to the White House and the National Security Council."

McCain weighed in on one other potential presidential candidate, Sen. Marco Rubio, R-Fla., with whom he has worked on immigration overhaul.

"Marco Rubio has played a tremendously important role, he is very articulate, and he has been going out there on the talk shows," McCain said of Rubio's role in the immigration debate. "He has a bright future in the party. He is a valuable candidate for president. Some people say this will set him back, but remember, I got the nomination of the party after failing on immigration in 2007."

And when it comes to his own legacy, McCain said there's one thing missing: "Am I happy? I'd like to be president of the United States."