Sen. John McCainJohn McCainBush biographer: Trump has moved the goalpost for civilized society White House to pressure McConnell on ObamaCare McCain: Trump needs to state difference between bigots and those fighting hate MORE (R-Ariz.) joked that it would be a tough decision to vote for Sen. Rand PaulRand PaulCurtis wins GOP primary for House seat vacated by Jason Chaffetz Glimmer of hope in bipartisan criminal justice reform effort Trump barrage stuns McConnell and his allies MORE (R-Ky.) over Democrat Hillary ClintonHillary Rodham ClintonAssange meets U.S. congressman, vows to prove Russia did not leak him documents High-ranking FBI official leaves Russia probe OPINION | Steve Bannon is Trump's indispensable man — don't sacrifice him to the critics 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 RubioScarborough: Trump has chosen the 'wrong side' THE MEMO: Trump reignites race firestorm RNC spokeswoman: GOP stands behind Trump's message 'of love and inclusiveness' 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 CruzThe media couldn't be more blatant in distorting Trump's words on Charlottesville Curtis wins GOP primary for House seat vacated by Jason Chaffetz Kimmel: Let’s make Trump a king so he has no power MORE (R-Texas) and Rep. Justin AmashJustin AmashThe five kinds of Republicans who could primary Trump Overnight Defense: Military won't lift transgender ban until Trump sends directions | House passes national security spending | Russian sanctions bill heads to Trump Overnight Finance: House passes spending bill with border wall funds | Ryan drops border tax idea | Russia sanctions bill goes to Trump's desk | Dems grill bank regulator picks 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.