Sen. Susan Collins Susan Margaret CollinsSenate GOP set to vote on Trump's Supreme Court pick before election Democratic senator to party: 'A little message discipline wouldn't kill us' Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy MORE (R-Maine) criticized both Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellHawley warns Schumer to steer clear of Catholic-based criticisms of Barrett Senate GOP set to vote on Trump's Supreme Court pick before election Harris slams Trump's Supreme Court pick as an attempt to 'destroy the Affordable Care Act' MORE (R-Ky.) and Democrats for weighing in on the impeachment trial, saying that senators shouldn't "prejudge" the evidence.

"It is inappropriate, in my judgment, for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us, because the each of us will take an oath, an oath that I take very seriously, to render impartial justice," Collins told Maine Public Radio, asked about McConnell's pledge to coordinate with the White House.

Collins singled out both McConnell and Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.), who is running for the Democratic presidential nomination, as two senators who have made comments that raise questions about their ability to be impartial during the Senate trial.

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"I have heard Democrats like Elizabeth Warren saying that the president should be impeached, found guilty and removed from office. I've heard the Senate majority leader saying that he's taking his cues from the White House. There are senators on both sides of the aisle, who, to me, are not giving the appearance of and the reality of judging that's in an impartial way," she said.

Collins, viewed as a crucial swing vote in the looming impeachment trial, is the second Republican senator who has raised concerns about McConnell's comments during the past week.

Sen. Lisa Murkowski Lisa Ann MurkowskiSenate GOP set to vote on Trump's Supreme Court pick before election Supreme Court fight pushes Senate toward brink Democratic senator to party: 'A little message discipline wouldn't kill us' MORE (R-Alaska) told an Alaska TV station that she was "disturbed" by the GOP leader's pledge of "total coordination."

McConnell has tied himself closely to President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE as he's plotted the party's impeachment strategy. He said during a Fox News interview earlier this month that he will be in "total coordination" with the White House. He also told reporters during a separate press conference that he was "not an impartial juror" in the looming trial.

The remarks have earned the GOP leader bipartisan flack from lawmakers, though his allies have been quick to note that some Democrats have already indicated they think Trump should be removed from office.

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Collins's comments to the Maine radio station on Monday mark a further distancing from McConnell's rhetoric. She initially told reporters in Washington that McConnell's pledge of coordination “would not be the approach that I’ve taken.”

Democrats are hoping to drive a wedge between potential swing Republicans, like Collins and Murkowski, and McConnell as they look to win over four GOP senators to support their call for witnesses and documents.

Collins told the Maine radio station that she was "open" to witnesses, but also appeared to endorse McConnell's floated structure saying that a decision on who, if anyone, should testify should wait until after both sides have presented their opening arguments.

Collins pointed to the impeachment trial of former President Clinton as a potential framework for the Trump trial. In 1999, the Senate passed a resolution in a 100-0 vote that established the process for a trial, and then passed a second resolution along party lines that called for closed-door depositions with three witnesses.

Senate Minority Leader Charles Schumer Chuck SchumerPelosi: Trump Supreme Court pick 'threatens' Affordable Care Act Will Democrats attempt to pack the Supreme Court again? Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' MORE (D-N.Y.) has said he wants the Senate to pass one resolution at the outset of the trial that would deal with both procedure and an agreement on specific witnesses.