Sen. Mike Lee is rebuffing demands to endorse Donald Trump that are coming from some conservative media personalities.

The Utah Republican is withholding support for Trump, citing the presumptive GOP presidential nominee's insufficient commitment to constitutional and conservative principles.

Making the rounds this week on television and talk radio to promote the release of the paperback edition of his book, Our Lost Constitution, Lee came under fire for refusing to back Trump.

Lee supported Sen. Ted Cruz of Texas during the primary.

After Trump essentially wrapped up the nomination, Lee raised eyebrows when he told constituents that he wasn't ready endorse, saying that the New York businessman "scares me to death."

The Washington Examiner first reported those comments, made in mid May.

Seven weeks later, some conservative media personalities are getting impatient. In interviews, they're pushing Lee to back Trump in his race against Democrat Hillary Clinton.

But Lee, a small-government, constitutionalist first elected in the 2010 Tea Party wave, is adamantly refusing. On Wednesday, Lee clashed with Newsmax television host Steve Malzberg during an interview conducted via telephone. Buzzfeed first reported on their exchange.

"I don't understand why you're not out there, Trumpeting Trump?" Malzberg asked, to which Lee responded: "Hey look Steve, I get it, you want me to endorse Trump." Said Malzberg: "Well, I don't understand why you're not."

At this point, Lee let loose.

"We can get into that if you want. We can get into the fact that he accused my best friend's father of conspiring to kill JFK," he said, referring to an attack Trump directed at Cruz during the primary. "We can go through the fact that he's made some statements that some have identified, correctly, as religiously intolerant. We can get into the fact that he's wildly unpopular in my state in part because my state consists of people who are members of a religious minority."

By the time Lee found himself embroiled in that explosive exchange, he had already sparred one day earlier with Newsmax host J.D. Hayworth, a former Arizona congressman. Hayworth asked Lee, essentially, why Clinton doesn't "scare you to death" more than Trump.

Lee answered that she does, but added that he remains concerned about Trump.

"I still do have some concerns. I have not endorsed him. I would like to see him moving forward with an aggressive agenda of constitutional reform, one that would focus on federalism and separation of powers," Lee said. "Mr. Trump has yet to do that."

Hayworth again pushed Lee, firmly, but politely, to explain his resistance to Trump and what it would take for him to relent. Hayworth argued that Trump's picks for the Supreme Court, for instance, would surely be more to Lee's liking than anyone nominated by Clinton.

At this point Lee appeared to grow somewhat agitated.

"Okay, look, I agree with you completely. I don't want Hillary Clinton as president. I don't want Hillary Clinton putting people on the U.S. Supreme Court," the senator said. "Look I get it, J.D., you want me to endorse Donald Trump — I get that. There are certain standards I employ when deciding whether or not I'm going to endorse a presidential candidate."

"I'm not going to endorse someone," Lee continued, "unless I believe that person believes as I believe in the Constitution. Unless that person feels passionately that we need to restore federalism and separation of powers. I have endorsed one presidential candidate this year, that was Ted Cruz. He believes in these things. If and when I see those things from Donald Trump, then I'll consider that. Until I see it, I'm not going to do it."

Prior to the Hayworth interview, the matter arose in a Monday radio interview with Sean Hannity.

The Fox News Channel host didn't criticize Lee specifically so much as express his general frustration with the Republicans in Washington who haven't enthusiastically backed Trump.

But Lee's aggressive response to Malzberg in particular makes more sense in the context of the fact that it came at the tail end of a week spent defending his skepticism of Trump and repeatedly explaining his refusal to endorse.

"I am angry that some Republicans like [House Speaker] Paul Ryan and [South Carolina Senator] Lindsey Graham seem to be doing everything they can do to sabotage Donald Trump's chances. Am I wrong?" Hannity asked Lee.

"Well, I do think, we need to focus, as a party, on a path toward coming together," Lee answered. "I understand there are differences of opinion within the party but we've got to find ways to reconcile those and to move forward."