Still, Romney isn’t going full Trump resistance: He knows his group can’t bring in Bolton alone without enraging some of his colleagues. So any successful effort to hear new testimony would also likely have to include witnesses whom Trump wants to subpoena too.

“My expectation is that were there to be that testimony from Mr. Bolton, that there would be testimony from someone on the defense side as well in order to get some 50-plus people to agree,” Romney said. “I’m not going to be counting noses as to who would support or not support that at this stage, but I may down the road.”

A former governor of Massachusetts, 2012 presidential nominee and wealthy businessman, the 72-year-old Romney makes an unlikely freshman senator. But he’s mostly fit in with his colleagues — even hosting the party’s informal dinner on Monday with helpings of Chick-fil-A.

Yet Romney does get a rise out of Republicans when he challenges the president. Romney's GOP colleagues remember his harsh rhetoric against Trump during the 2016 campaign, though tensions have ebbed and flowed in the past four years.

“I’d rather he not” push for witnesses, said Sen. Jim Inhofe (R-Okla.). “He isn’t all that close to the administration. … I don’t agree with him.”

Sen. David Perdue (R-Ga.) has compared Romney to “Jeff Flake on steroids,” and Sen. Rick Scott (R-Fla.) asserted last fall that Romney “thinks the worst of the president.” Trump himself called Romney a “pompous ass” when he expressed concerns about asking other countries to investigate Joe Biden.

A senior administration official acknowledged Bolton’s book could hurt the GOP’s efforts to block witness testimony but said it wasn’t because of anything Romney is doing.

“He’s doing what he’s already doing. It’s personal" between him and Trump, the official said.