One House Democrat's accusation amid the Trump impeachment trial “stunned” even the most moderate of Senate Republicans, reportedly prompting GOP Sen. Susan Collins to write a note to Chief Justice John Roberts about decorum on the floor of the upper chamber.

Collins, R-Maine, is the latest to signal her concerns, after impeachment manager Rep. Jerrold Nadler, D-N.Y., asserted earlier this week that GOP senators were engaged in a “cover-up.”

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Collins told Politico she was “stunned” by Nadler’s comments, and confirmed to the outlet that she wrote a note that made its way to Roberts after a tense back and forth between Nadler and White House Chief Counsel Pat Cipollone.

“It reminded me that if we were in a normal debate in the Senate, that the rule would be invoked to strike the words of the senator for impugning another senator,” she told Politico. “So, I did write a note raising the issue of whether there’d been a violation of the rules.”

Collins said she gave the note to the secretary for the majority, Laura Dove, and “shortly thereafter, the chief justice did admonish both sides. And I was glad that he did.”

CNN first reported that Collins wrote a note for the chief justice.

“I’m going to cast my vote regardless of the congressman’s performance,” she told Politico. “His negative comments about the Senate will not have any impact.”

Collins is among a small group of Republican senators seen as a possible swing vote on impeachment and a possible supporter of allowing more witness testimony later on in the trial.

Also included in that group is Sen. Lisa Murkowski, R-Alaska, who said this week she was offended by Nadler’s presentation.

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“I took it as offensive,” she told reporters Wednesday. “As one who is listening attentively and working hard to get a fair process, I was offended.”

Sen. Josh Hawley, R-Mo., said Democrats took “a huge step backward” in their opening.

“I mean, that’s an extraordinary thing to say on the floor of the United States Senate, the middle of the trial, and that’s what drew the rebuke and rightly so,” Hawley said. “I can tell you, there was an open, open gasping on the Senate floor when Nadler was saying these things. I mean, It’s really, really extraordinary.”

FILE - In this Nov. 6, 2019, file photo, Sen. Susan Collins, R-Maine, is surrounded by reporters as she heads to vote at the Capitol in Washington. U.S. Sen. Collins officially launched her bid for a reelection Wednesday, Dec. 18, setting up an expensive and closely watched battle for the seat the moderate Republican from Maine has held for nearly 24 years. (AP Photo/J. Scott Applewhite, File)

Nadler’s opening statement began by calling Republican support for the president “embarrassing.”

“The president is on trial in the Senate, but the Senate is on trial in the eyes of the American people,” he said. “Will you vote to allow all the relevant evidence to be presented here? Or will you betray your pledge to be an impartial juror?”

“Will you bring Ambassador [John] Bolton here? Will you permit us to present you with the entire record of the president's misconduct? Or will you instead choose to be complicit in the president's cover-up?” he continued. “So far I'm sad to say I see a lot of senators voting for a cover-up, voting to deny witnesses, an absolutely indefensible vote, obviously a treacherous vote.”

The Trump legal team attempted to defend against Nadler’s remarks, drawing the chief justice to issue a rebuke to both sides.

"It is appropriate at this point for me to admonish both the House managers and the president's counsel in equal terms to remember that they are addressing the world's greatest deliberative body," Roberts said. "One reason it has earned that title is because its members avoid speaking in a manner, and using language, that is not conducive to civil discourse. "

Roberts continued: "In the 1905 [Judge Charles] Swayne trial, a senator objected when one of the managers used the word 'pettifogging' -- and the presiding officer said the word ought not to have been used. I don't think we need to aspire to that high a standard, but I do think those addressing the Senate should remember where they are."

Meanwhile, senators, in general, have already shown some restlessness on the floor after hours and hours of opening statements from the prosecution.

“The House is completely miscalculating how to handle this,” a Senate GOP source told Fox News Thursday. “They’re putting far too much emphasis on the time they use than the substance of what they’re delivering.”

Fox News' Jason Donner, Gregg Re and Marisa Schultz contributed to this report.