Republican Sen. Rand Paul said he is opposed to calling former national security adviser John Bolton as a witness in President Trump’s impeachment trial.

“He’s a disgruntled employee with an ax to grind,” the Kentucky senator, who has long been critical of Bolton’s stances, told the Washington Post.

Trump told reporters in Davos, Switzerland, on Wednesday that testimony from certain administration officials, including Bolton, could threaten national security.

Democrats have pushed for Bolton, who left the White House in September after clashing with Trump on foreign policy issues, to testify about Trump’s conduct in the Ukraine affair. Trump said he “would rather” have Bolton appear before the Senate, but “the problem with John is that it’s a national security problem.”

“He knows some of my thoughts. He knows what I think about leaders. What happens if he reveals what I think about a certain leader, and it’s not very positive, and then I have to deal on behalf of the country? It’s going to be very hard. It’s going to make the job very hard. He knows other things. And I don’t know if we left on the best of terms. I would say probably not,” Trump said.

Bolton’s former aide, Fiona Hill, testified to the House that Bolton opposed the Ukraine pressure campaign, saying he wanted no part of what he described as a "drug deal" that other administration officials were "cooking up."

He is reportedly writing about "at least some of what he saw" in regards to Trump pressuring Ukraine to investigate Joe Biden and his son Hunter in his forthcoming book.

“Some people who have talked to him think he has an ax to grind, that he’s angry he was publicly fired by the president. But he also has a history of believing in unlimited powers for the president. Which is the guiding light for John Bolton at this point? Ax to grind and books to sell? Or be a player and say, 'Even if I’m gone, he’s doing what I want on Iran and other things?’” Paul said.