WASHINGTON — Senator Susan Collins, Republican of Maine, criticized some of her Senate colleagues, including the majority leader, for appearing to “prejudge the evidence” in impeachment proceedings against President Trump, becoming the second Republican senator to question Senator Mitch McConnell’s pledge to coordinate with the White House.

Impeachment rules require a simple majority vote, meaning Mr. McConnell, Republican of Kentucky, can afford to lose only four members of his conference if he is to set the parameters of a trial. Senator Lisa Murkowski, Republican of Alaska and another moderate with an independent streak, said last week that she was “disturbed” by Mr. McConnell’s promise to work with the White House counsel to set the terms of the trial.

Both Senators Murkowski and Collins also offered noncommittal positions on calling witnesses to an impeachment trial, which Democrats have pushed and Mr. McConnell has resisted. And both women questioned why the House did not go to court when administration officials ignored subpoenas. (Democrats, who control the House, have asserted that going to court to compel testimony from administration officials would take too long when the 2020 election is already in danger.)

“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,” Ms. Collins said in an interview with Maine Public Radio that was broadcast Monday. “That’s what it says, impartial justice.”