Donald J. Trump on Friday rejected an offer to debate Bernie Sanders before the June 7 California primary, saying, “It seems inappropriate that I would debate the second-place finisher” in the Democratic nominating contest.

The two men had been kicking around the idea of a debate since Wednesday, when Mr. Trump appeared on “Jimmy Kimmel Live” and Mr. Kimmel said that Mr. Sanders had passed along an invitation to debate. Mr. Trump, who secured the Republican Party’s nomination for president on Thursday, told Mr. Kimmel he was open to the idea.

But after Mr. Sanders pressed the issue on Friday, Mr. Trump released a statement batting away any debate with him.

“Based on the fact that the Democratic nominating process is totally rigged and crooked Hillary Clinton and Deborah Wasserman Schultz will not allow Bernie Sanders to win, and now that I am the presumptive Republican nominee, it seems inappropriate that I would debate the second-place finisher,” Mr. Trump said.