Sen. John McCainJohn McCainOvernight Cybersecurity: Bad Russian intel may have swayed Comey's handling of Clinton probe | Apple sees spike in data requests | More subpoenas for Flynn | DOJ's plan for data warrants McCain: Dems killed Lieberman’s FBI shot When education is an ignored national security matter MORE (R-Ariz.) joked that it would be a tough decision to vote for Sen. Rand PaulRand PaulPaul: 0B Saudi arms deal ‘a travesty’ Senate feels pressure for summer healthcare vote Overnight Defense: Trump budget gets thumbs down from hawks | UK raises threat level after Manchester attack | Paul to force vote on 0B Saudi arms deal MORE (R-Ky.) over Democrat Hillary ClintonHillary Rodham Clinton5 things to watch in Montana’s special election Hannity on attempted advertiser boycott: 'Nobody tells me what to say on my show' Overnight Cybersecurity: Bad Russian intel may have swayed Comey's handling of Clinton probe | Apple sees spike in data requests | More subpoenas for Flynn | DOJ's plan for data warrants MORE if both won their party’s presidential nomination in 2016.

“It’s gonna be a tough choice,” McCain said with a laugh when asked about it in an interview with the New Republic published Tuesday. 

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McCain clarified that he believes Paul is trying to expand the GOP brand, and that he represents the same libertarian-leaning part of the party once represented by his father, former Rep. Ron Paul (R-Texas).

However, McCain heaped praise on another prospective Republican presidential candidate in 2016, Sen. Marco RubioMarco RubioMcConnell on Trump: 'We could do with a little less drama' Taking the easy layup: Why brain cancer patients depend on it Rubio: 'All options should be on table' if Flynn refuses new subpoenas MORE (Fla.). He called Rubio a “valuable candidate for president” and said his push for immigration reform will not impede his presidential prospects, harkening back to his own failed bid in 2008.

“He has a bright future in the party,” he said. “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.”

McCain has had numerous dustups this year with the Tea Party-fueled wing of the GOP this year. He opposed Paul’s filibuster of CIA director John Brennan’s nomination over the U.S. use of drones, and described Paul, Sen. Ted CruzTed CruzGOP senators distance themselves from House ObamaCare repeal bill Cruz: Jokes about me in Franken's book 'obnoxious' The Hill's 12:30 Report MORE (R-Texas) and Rep. Justin AmashJustin AmashGOP lawmaker backs Dem push for Trump tax returns The Hill's 12:30 Report The Memo: GOP talk of impeachment highlights Trump’s troubles MORE (R-Mich.) as “wacko birds.”

In the interview, he said the isolationist wing of the GOP has been strengthened by the recession.

“There was an isolationist wing that fought against Reagan. And now the bad economy has exacerbated what has always been out there,” he said.

McCain attempted to separate Paul’s brand of conservatism from his former running mate in 2008, Sarah Palin, arguing her message was positive.

“They came to power to keep things from getting done,” McCain said.

Reflecting on his own legacy, he said he has had the best ride, but still expressed some regret from his failed 2008 bid for president.

“Am I happy? I’d like to be president of the United States,” he said.