Donald J. Trump has asked Republicans who do not have anything nice to say about him to not say anything at all.

But Senator Mike Lee, the Republican from Utah, could not help himself.

Mr. Lee, a close friend of Mr. Trump’s former primary opponent, Senator Ted Cruz, let his true feelings be known on Wednesday night in an interview with Newsmax when he was asked why he had not endorsed the presumptive Republican presidential nominee. Faced with the prospect of a Hillary Clinton presidency, how could he not?

“Well, we can get into that if you want,” Mr. Lee said. “I mean we can get into the fact that he accused my best friend’s father of conspiring to kill J.F.K.”

The comment was a reference to Mr. Trump’s insinuation that Mr. Cruz’s father was involved with the assassination of President John F. Kennedy in 1963.