(CNN) Sen. Mike Lee on Wednesday cited Donald Trump's use of a baseless tabloid report that Ted Cruz's father helped conspire to kill President John F. Kennedy as a reason he has yet to endorse the presumptive Republican nominee in a heated exchange.

"I mean we can get into the fact that he accused my best friend's father of conspiring to kill JFK," the Utah Republican said told Newsmax's Steve Malzberg , when asked about endorsing Trump, according to a transcript posted by Breitbart

Lee delivered a passionate rebuttal to rounds of criticism within the GOP by those who have backed Trump toward those who still haven't gotten on board, chiding critics "don't sit here and tell me that I have no reason to be concerned about Donald Trump" while saying he needs "assurances" that Trump would not act as an "authoritarian" or "autocrat."

"We can go through the fact that he has made some statements that some have identified correctly as religiously intolerance. We can get into the fact that he is so unpopular because my sate consists of members who were a religious minority church," he said.

Lee added: "Don't sit here and tell me, Steve, that I have no reason to be concerned about Donald Trump."

Malzberg countered by criticizing presumptive Democratic nominee Hillary Clinton, saying concerns about the former secretary of state connected to her email server and the Clinton Foundation should "dwarf" those about Trump.

The Utah senator blasted Trump again for the JFK assassination rumors.

"He said that. He actually said that -- without a scintilla of evidence. That concerns me. And again, I hope I can get over this, because I can't vote for Hillary," he said. "What I am saying is Donald Trump can still get a vote from a lot of conservatives like me, but I would like some assurances on where he stands. I would like some assurances that he is going to be a vigorous defender of the U.S. Constitution. That he is not going to be an autocrat. That he is not going to be an authoritarian. That he is not somebody who is going to abuse a document that I have sworn an oath to uphold and protect and defend."

"I am sorry, sir, but that is not an unreasonable demand," Lee finished.

Trump came in third in Utah's Republican caucuses, earning just 14% support.