Trump warns McCain: 'At some point I fight back, and it won't be pretty'

President Trump warned on Tuesday that he would "fight back" after Sen. John McCainJohn Sidney McCainGabbard hits back at Meghan McCain after fight over Assad Mellman: Where are good faith and integrity? GOP senator says Republicans didn't control Senate when they held majority MORE (R-Ariz.) delivered a blunt denunciation of nationalist forces that was seen by many as a thinly-veiled attack on the president.

"People have to careful, because at some point I fight back," Trump told WMAL radio host Chris Plante. "I'm being very nice. I'm being very, very nice. But at some point I fight back, and it won't be pretty."

McCain responded to Trump's comments moments later, saying that he has "faced far greater challenges than this," according to CNN. 

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Speaking at an award ceremony in Philadelphia Monday night, McCain warned against "spurious nationalism" and the dangers of the U.S. abdicating its global leadership responsibilities. 

"To fear the world we have organized and led for three-quarters of a century, to abandon the ideals we have advanced around the globe, to refuse the obligations of international leadership and our duty to remain 'the last best hope of Earth' for the sake of some half-baked, spurious nationalism cooked up by people who would rather find scapegoats than solve problems is as unpatriotic as an attachment to any other tired dogma of the past that Americans consigned to the ash heap of history," McCain said at the National Constitution Center, where he was awarded the Liberty Medal.

The remark was seen by many as a jab at Trump, who campaigned on an "America first" agenda and has scorned international agreements and institutions as unfair to the U.S.