But for a guy who says he thinks has "been losing so long he doesn't know how to win anymore," Trump has certainly given the Arizona Republican plenty of campaign cash.
Federal Election Commission records show Trump has donated tens of thousands of dollars to McCain's senatorial re-election campaigns and to his 2008 presidential bid directly through campaign and political action committees.
Trump's latest pop-off came after McCain, a Navy veteran who endured torture during his more than five years as a prisoner of war in Vietnam, criticized him for a raid in Yemen that left Navy Seal William "Ryan" Owens dead.
The new president fielded sharp blowback during the 2016 campaign after sneering that McCain wasn't "a war hero because he was captured. I like people who weren't captured." Trump himself received one medical and four education-related deferments from military service in the Vietnam era.
This isn't the first time a former Trump beneficiary has been the target of his ire: Before labeling former Rep. Anthony Weiner a "perv," Trump had also donated generously to the political ambitions of the estranged husband of Huma Abedin, Hillary Clinton's longtime top aide.
Correction: Feb. 9, 2017
An earlier version of this article misstated the reasons for Trump's deferments from military service. He received one medical and four education-related deferments.