President Donald Trump responded on Tuesday to criticism from former failed presidential candidate Sen. John McCain, warning that he might be prepared to go to war with the ailing senator.

Trump’s remarks, given in an interview with WMAL’s Chris Plante, were in response to the speech McCain delivered at the National Constitution Center Liberty Medal ceremony in Philadelphia.

“People have to be careful because at some point, I fight back,” Trump said. “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 alluded to Trump’s foreign policy as “half-baked, spurious nationalism” and said it was “unpatriotic.”

Since the veteran Arizona senator was diagnosed with a deadly form of brain cancer in July, he has opted to be more vocal in his opposition to Trump.