President Donald Trump on Tuesday fired back at Sen. John McCain (R-AZ), who on Monday delivered a speech slamming Trump’s brand of “half-baked nationalism.”

Appearing on talk radio station WMAL in Washington DC, Trump was asked about the criticism McCain leveled at him this week, in which he lashed out at “half baked, spurious nationalism” that’s being “cooked up by people who had rather find scapegoats than solve problems.”

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

Trump has regularly attacked McCain after the senator was a key vote against the Republican efforts to repeal and replace the Affordable Care Act this past summer. In addition to McCain, Trump has also regularly picked fights with Sens. Bob Corker (R-TN) and Jeff Flake (R-AZ).