Utah Sen. Mike Lee did not mince words when he was asked why he has not yet endorsed Donald Trump for president.

“We can get into that if you want,” Lee said in a telephone interview with Newsmax TV on Wednesday. “We can get into the fact that he accused my best friend’s father of conspiring to kill JFK.”

But Lee wasn’t done there. He continued: “We can go through the fact that he’s made 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 church — a people who were ordered exterminated by the governor of Missouri in 1838. And statements like that make them nervous.”

During the Republican primary, Trump suggested that rival Texas Sen. Ted Cruz’s father was somehow connected to the assassination of President John F. Kennedy. Trump’s claim was based on a dubious National Enquirer story. Lee endorsed Cruz in the race.

“He said that. He actually said that,” Lee recalled on Wednesday. “He said that without any scintilla, without a scintilla of evidence. Now that concerns me.”

Nonetheless, the Utah Republican left the door open for a possible Trump endorsement if he “heard the right things out of him.”

“I can’t vote for Hillary,” Lee said of the presumptive Democratic nominee. “I know there’s no possibility of that. What I am saying is that Donald Trump can still get a lot of votes from a lot of conservatives like me, but I would like some assurances on where he’s going to stand his ground. I’d like some assurances that he’s going to be a vigorous defender for the U.S. Constitution.”

It appears Lee is not alone. According to a Fox News national poll released Wednesday, more than half of Republicans say they would prefer a different nominee than Trump. And just 74 percent said they preferred him over Hillary Clinton, down from 82 percent in May.

The survey showed that Trump trailed the former secretary of state by 6 percentage points (44 percent to 38 percent) in a general election matchup. In early June, the same poll had Clinton leading Trump by just 2 points (42 percent to 39 percent).

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Trump’s slump comes after criticism from conservatives over his response to the June 12 mass shooting at a gay nightclub in Orlando, and his attack on a U.S.-born judge whose parents are Mexican immigrants.

And while Clinton has been dogged by polls consistently showing most Americans don’t trust her, the Fox News survey showed that just 34 percent view Trump as honest and trustworthy, compared to 40 percent in May.

In an interview with Fox host Bill O’Reilly Wednesday night, Trump dismissed the idea he can’t be trusted.

“I’m honest, I’m trustworthy. I tell it like it is,” he said. “I’m here, I didn’t need to do this. I’m enjoying it, but I’m enjoying it because we are going to make America great again. We are going to make America great again. We’re also going to make America safe again.”