Sen. John McCainJohn Sidney McCainGOP strategist donates to Alabama Democrat Meghan McCain knocks Bannon: 'Who the hell are you' to criticize Romney? Dems demand Tillerson end State hiring freeze, consult with Congress MORE (R-Ariz) on Friday said Sens. Ted CruzRafael (Ted) Edward CruzDebbie Wasserman Schultz marks 10 years as breast cancer survivor Foreign agent registration is no magical shield against Russian propaganda Let Trump be Trump and he'll sail through 2020 MORE (Texas) and Rand PaulRandal (Rand) Howard PaulLexington mayor launches bid for Congress Trump-free Kennedy Center Honors avoids politics Meet the Iran hawk who could be Trump's next secretary of State MORE (Ky.) would be viable presidential nominees for the GOP in 2016.

McCain has had dustups with the two conservative firebrands throughout the year, at one point colorfully labeling them "wacko birds." 

Both Cruz and Paul are eyeing runs for the White House in 2016, and would have a real shot if nominated, according to McCain.

“If that is who the party chooses I am sure that that nominee will be viable,” he said in an interview with ABC published Friday.

McCain warned, however, that any GOP candidate would founder in a general election if immigration reform stalls in Congress. Both Cruz and Paul voted against a comprehensive immigration reform bill in the Senate earlier this year. 

“I want to say in the strongest possible terms that if we fail on immigration reform, it won’t matter who our nominee is because of the polarization of the Hispanic vote,” he said. 

He also had some advice for Cruz, who earlier in the week claimed some GOP colleagues had not signed onto his proposal to tie a deal to avert a government shutdown to the defunding of ObamaCare because they were “scared” of being beaten up politically.

McCain said he has seen his share of partisan battles but tries to keep his attacks tied to issues. He said Republicans should be aware of how they treat their colleagues. 

“Look, I have been as ferocious a fighter and I think as partisan as anybody but I really try hard not to get personal,” he said. “Debate on the issue as hard as you can but don’t say your opponents, the people who disagree with you, are scared. It has been a long time since I’ve been scared.”