GOP Sen. Susan Collins said she’s open to calling additional witnesses for the Senate trial of President Donald Trump’s impeachment, but stopped short of backing demands from Democrats for testimony to be set before the process begins.

The Maine Republican is considered a possible swing vote on deciding the rules for the Senate trial when it gets underway, likely next month. While Trump is all but certain to be acquitted in the Republican-majority Senate, it would only take four Republicans to side with Democrats to call additional witnesses who weren’t part of the House impeachment inquiry.

Collins and Alaska Sen. Lisa Murkowski, another potential Republican wild card, both have expressed concern about the impartiality of their colleagues, including Senate Majority Leader Mitch McConnell’s statements that he is coordinating closely with the White House on trial procedures.

“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,” Collins said on Maine Public Radio.

Murkowski last week told KTUU-TV of Alaska she was “disturbed” by McConnell’s comments and that Republicans should “take that step back from being hand in glove with the defense.”

However, both indicated support for using the 1999 impeachment trial of then-President Bill Clinton as a template, where a decision on witnesses is left until after opening arguments by the House impeachment managers and the president’s counsel.

That’s been McConnell’s position as he’s rebuffed Democratic leader Chuck Schumer’s demand that Secretary of State Michael Pompeo, Acting White House Chief of Staff Mick Mulvaney and former National Security Adviser John Bolton, among others, be called as witnesses even though they refused to testify in the House inquiry.

“I am open to witnesses. I think it’s 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,” said Collins, who is up for re-election in November and is one of the top targets of Democrats seeking to flip control of the Senate.

Collins, Murkowski and Utah Sen. Mitt Romney are viewed by Democrats as the Republicans who may be most willing to buck party leadership on the impeachment trial rules. They likely also would seek to pressure Cory Gardner of Colorado and Martha McSally of Arizona, who have been strong Trump supporters but face tough 2020 re-election fights.