Republican Sen. Mitt Romney is not keen on President Trump’s attacks on impeachment witnesses but conceded that the insults are working.

“He has made a career of trashing people by tweet,” Romney, 72, told the New York Times.

“It is part of his shtick and it’s working for him,” the Utah senator said of Trump’s attacks on his former national security adviser, John Bolton. “I think it’s unfortunate.”

In recent days, Trump, 73, has trashed Bolton, who has undermined the president's and his legal team’s claim that the freeze of security aid to Ukraine was not connected to a pressure campaign to get the country to investigate 2020 Democratic candidate Joe Biden. Trump also lashed out at witnesses in the House impeachment investigation.

Bolton, 71, made the allegations in his forthcoming book, bolstering calls for him to testify in the Senate trial. The Senate is expected to vote Friday on whether to call witnesses in the trial. Most Republicans are not in favor of having witness testimony ahead of a vote on whether to acquit or convict Trump on obstruction and abuse of power charges.

Trump has argued that Bolton’s allegations contain nothing of substance but has also asserted that any possible testimony from him would threaten national security.