Two key moderate Senate Republicans criticized House Judiciary Committee Chairman Jerry Nadler, D-N.Y., on Thursday after he claimed this week that senators who do not support hearing from witnesses and entering documents into President Donald Trump's impeachment trial would be complicit in a cover-up.

An aide told NBC News that Sen. Lisa Murkowski of Alaska was "offended" when Nadler, one of the seven House impeachment managers, made the accusation on the Senate floor.

Murkowski is one of a handful of moderate GOP senators who have expressed an openness to calling witnesses, including top Trump administration officials. For Democratic senators who need four Republican votes to support them on the issue, Murkowski's would be a critical vote.

Susan Collins of Maine, another Republican who has been open to witnesses, wrote a note to Chief Justice John Roberts, who is presiding over the trial, taking issue with Nadler's remark, according to a spokesperson.

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"Well, I was stunned by Congressman Nadler's approach, and it reminded me that if we were in a normal debate in the Senate that the rule will be invoked to strike the words of the Senator, for imputing another Senator in this case," she said in a statement.

"So, I did write a note raising the issue of whether there had been a violation of the rules of the Senate, and I gave that note to Laura Dove, and well shortly thereafter the Chief Justice did admonish both sides and I was glad that he did," she added.

Dove is the Senate majority secretary.

Despite her objection to the remark, Collins said it would not affect her votes during the trial.

Nadler criticized Republicans on Wednesday shortly after midnight toward the end of a marathon day of debate over the rules of the impeachment trial. Nadler's comments came as he was making the case to call former national security adviser John Bolton to testify. Other top Democrats also excoriated Republicans for being unwilling to have new witnesses and documents in the trial.

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

"Will you bring Ambassador Bolton here? Will you permit us to present you with the entire record of the president's misconduct?" he asked. "Or will you instead choose to be complicit in the president's cover-up? 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."

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Other Republican senators, such as Ron Johnson of Wisconsin and Lindsey Graham of South Carolina, chairman of the Judiciary Committee, also criticized Nadler's comment.

"If the Democrats are smart, they won't put Jerry Nadler on the field again," Johnson said. "He was so out of line. It's offensive accusing us of a cover-up."