Sen. Susan Collins (R-Maine) said she is open to calling witnesses in the impeachment trial of President Donald Trump . However, she added that it is too early to determine which witnesses should appear and that the Senate ought to decide after opening arguments and initial questioning of both sides.

I am open to witnesses. I think its premature to decide who should be called until we see the evidence that is presented and get the answers to the questions that we senators can submit through the chief justice to both sides, Collins told Maine Public Radio on Monday.

Collins stance on the parameters of the impeachment trial aligns her with Senate Majority Leader Mitch McConnell (R-Ky.), who has all but promised a swift acquittal of Trump. McConnell has argued for following the framework of the 1999 impeachment trial of President Bill Clinton, which punted a decision on witness testimony until after the initial arguments and senatorial questioning.

We havent ruled out witnesses, McConnell said last week in a Fox & Friends interview. Weve said lets handle this case just like we did with President Clinton. Fair is fair.

Senate Minority Leader Chuck Schumer (D-N.Y.), meanwhile, is demanding the Senate agree upfront to hear from witnesses who refused to appear during House committee hearings, including acting White House Chief of Staff Mick Mulvaney and former national security adviser John Bolton. The Democratic leader argued that witnesses with firsthand knowledge about Trumps decision to block military assistance to Ukraine could provide crucial information.

President Trump, if you are so confident you did nothing wrong, why wont you let your men testify? Schumer said at a press conference in New York City on Monday. What are Senator McConnell and President Trump afraid of if all the facts come out?

House Speaker Nancy Pelosi (D-Calif.) has delayed sending the articles of impeachment to the Senate, reportedly to give Schumer more leverage in talks with McConnell over procedures for a fair trial. The Senate cannot begin the trial until the House formally transmits the articles to the upper chamber. But just days before senators are due to return to Washington from their holiday break, there are no signs the impasse between the two Senate leaders is easing.

In her interview with Maine Public Radio, Collins also called McConnells vow of total coordination with the White House on the trial process inappropriate, while chiding some Senate Democrats for rushing to judgment.

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 each of us will take an oath, an oath that I take very seriously, to render impartial justice, she said.

And I have heard Democrats like Elizabeth Warren , saying that the president should be impeached, found guilty and removed from office, Collins added. Ive heard the Senate majority leader saying that hes 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 this in an impartial way.

McConnell told reporters earlier this month that he didnt view himself as an impartial juror. I would anticipate we will have a largely partisan outcome in the Senate, he said. Im not impartial about this at all.

Sen. Lisa Murkowski (R-Alaska) was the first Republican to publicly voice concern over the Senate majority leaders handling of the proceedings, telling KTUU last week that she was disturbed by McConnells comments.

Democrats in the Senate want to hear from White House officials who, at Trumps urging, defied House subpoenas and didnt testify. The list includes Mulvaney; Bolton; Robert Blair, senior adviser to Mulvaney; and Michael Duffey, associate director for national security at the Office of Management and Budget.

To subpoena their desired witnesses, Democrats will need to convince at least four Republican senators to vote with them. Moderates like Collins and Murkowski will be their top targets.

Collins is seeking reelection in 2020 in what is shaping up to be a fierce contest.