WASHINGTON (Reuters) - Representative Adam Schiff, making his closing argument in the impeachment trial of U.S. President Donald Trump on Friday, seemed to have the Senate chamber more or less rapt, with lawmakers listening respectfully, whether they agreed with his arguments or not.

Then the Democratic congressman mentioned a CBS News report about a Trump confidante suggesting serious ramifications for Republican senators if they voted against the president, and the mood on the Republican side of the aisle shifted. Dramatically.

“Not true,” Senator Susan Collins of Maine declared out loud as fellow Republicans responded audibly and disgustedly at Schiff’s remarks.

Collins is a moderate Republican whose support Democrats are trying to win. Her response and that of her colleagues was all the more notable because senators had been warned not to talk during the proceedings or face potential imprisonment.

But Schiff’s remarks had touched a nerve. Leading the prosecution of Trump for Democrats from the House of Representatives, Schiff had cited a report suggesting that Republicans, known as members of the Grand Old Party (GOP), had been threatened.

“CBS News reported last night that a Trump confidante said that GOP senators were warned: vote against your president ... and your head will be on a pike,” Schiff said.

“I hope it’s not true. But I was struck by the irony of the idea, when we’re talking about a president who would make himself a monarch, that whoever that was would use the terminology of a penalty that was imposed by a monarch, a head on a pike.”

Republicans said they were insulted.

“The whole room was visibly upset on our side of it. It’s insulting and demeaning to everyone to say that we live in fear and that the president has threatened all of us,” Republican Senator James Lankford of Oklahoma told reporters later. “Nothing like going through three days of frustration and then cap it off with an insult.”

Republicans control the U.S. Senate and are not expected to convict and remove Trump from office in the impeachment trial. Many have complained that Schiff and other prosecuting lawmakers, known as managers in the trial, had presented a repetitive and unconvincing case against Trump.

If they needed more fodder, they got it on Friday, even though Schiff made clear he did not know if the CBS report had been true.

Democrats have accused Trump of being an authoritarian and charged him with abusing his office by pressuring Ukraine to investigate former Vice President Joe Biden, a 2020 Democratic presidential contender, and then obstructing Congress’ inquiry into the matter by barring witnesses and withholding documents.

Trump’s lawyers will begin presenting his defense in the trial on Saturday.