Kristol has long criticized Trump and led a movement encouraging an independent, third-party candidate to enter the presidential race and bring down the businessman. At least one person Kristol courted, retired Gen. James Mattis, came out and said he would not launch an independent bid.

Trump said he almost felt bad for Kristol before launching into an imitation of him: "We’re still looking for a third party candidate!"

Trump mocked Kristol, who he repeatedly referred to as working for "that magazine," for searching "for like nine months" for someone to challenge him.

"He can’t find anybody! What a loser!" Trump said. "By the way, I heard his magazine is doing so badly it’s going to fold."

Trump also questioned why television producers kept inviting Kristol on, given his history of making wildly off-base predictions.