Sen. Susan Collins, R-Maine, criticized Republicans and Democrats — citing Sens. Mitch McConnell and Elizabeth Warren by name — for making comments about the Senate impeachment trial of President Donald Trump before it has even started.

Collins, a moderate Republican who faces a tough re-election battle next year, said in a radio interview on Monday that it was "inappropriate" for McConnell to say he was working in "total coordination" with the White House and she excoriated Democrats for prejudging the process.

"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.

"I have heard Democrats like Elizabeth Warren saying that the president should be impeached, found guilty and removed from office," she continued. "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."

Collins is the second GOP senator to criticize McConnell's comments. Earlier this month, Sen. Lisa Murkowski of Alaska said she was "disturbed" that the Senate majority leader said he was coordinating with the White House.

Collins also told Maine Public Radio that she is "open" to calling witnesses in the Senate trial but stopped short of saying which administration officials should testify.

"I am open to witnesses," she said. "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," Collins when asked if key White House officials, such as acting chief of staff Mick Mulvaney and former national security adviser John Bolton, should appear.

Collins said Senate leaders should look to the 1999 impeachment trial of President Bill Clinton as a blueprint. She, who said she has been studying the trial and process, served as a juror at that time.

Senate Minority Leader Chuck Schumer, D-N.Y., held a press conference on Monday in New York and renewed his call for witnesses and documents after a New York Times report on Sunday detailed how key aides handled Trump's request to hold military aid from Ukraine.

"This story makes the choice even clearer: Will the Senate hold a fair trial, or will it enable in a cover-up?" Schumer said. "What are Senator McConnell and President Trump afraid of if all the facts come out?"