In an interview with Breitbart published this week, Mr. Trump said he had not decided whether he would join the debate, which is to be hosted by Fox News on Aug. 23 in Milwaukee, Wisconsin. According to the outlet, the interview was taped on July 27.

"I haven't totally made a decision. I like the debates. I might be here because of the debates. I might have won against Hillary [Clinton] with the debates, and I might have gotten the nomination because of the debates," Mr. Trump said.

"But when you're leading by 50 and 60 points against these people, and you have people at zero, and 1 and 2—and then they're going to be asking me hostile questions, and they probably won't have much of an audience if I'm not in the debates according to what I read," Mr. Trump continued. "If I'm not in the debates, then they're not going to have a very big audience. It seems almost like it would be foolish to do them."