Sen. Susan Collins, R-Maine, said Friday that she's been working with "a fairly small group" of Republican senators to make sure witnesses can be called in President Donald Trump's impending Senate impeachment trial.

"We should be completely open to calling witnesses," Collins told reporters in Bangor, Maine, the Bangor Daily News reported. She declined to say who or how many GOP lawmakers she's been working with, but said she was "hopeful that we can reach an agreement on how to proceed with the trial that will allow the opportunity for both the House and the president's counsel if they choose to do so."

Collins echoed those comments in a statement to NBC News and said, “It is important that both sides be treated fairly.”

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Senate Majority Leader Mitch McConnell, R-Ky., has said he plans to follow the model that was used in President Bill Clinton's impeachment trial. In that case, the two sides made opening arguments and took questions from senators before there was a motion to either dismiss the case or hear from witnesses. In the Clinton case, the motion to dismiss failed, and senators wound up taking videotaped depositions of three witnesses, portions of which were played at the trial.

McConnell has said he's working in "total coordination" with the White House on the trial, leading Democrats to charge that the majority leader plans to hold "sham" proceedings. Numerous Republican senators have said they don't see a need for witnesses to testify, despite former national security adviser John Bolton's recent offer to do so if subpoenaed.

Collins, a moderate Republican who is up for re-election this year, has criticized McConnell for his comments about working with the White House, and previously said she's "open" to calling witnesses. Sen. Mitt Romney, R-Utah, has said he'd "love to hear" what Bolton has to say. Bolton's lawyer has said he has pertinent information about the White House's dealings with Ukraine that has not been disclosed publicly.

One person who doesn't want to hear from Bolton is the president who fired him (although Bolton maintains he resigned). In an interview with Fox News on Friday, Trump said he'd likely invoke executive privilege to limit his testimony.

"I think you have to, for the sake of the office," the commander-in-chief said, maintaining it was important for "future, I'm talking about future, many future presidents."

In her statement, Collins suggested she was fine with McConnell's call to follow the Clinton model, as long as it allows for witnesses. "I have had many discussions with some of my Republican colleagues on how we can adhere as closely as practical to the precedent for conducting the impeachment trial of President Clinton, which included as a third stage the decision on whether to call witnesses," she said.

Other moderate Republicans have sidestepped questions about potential witnesses, but just a handful peeling away could be the deciding factor. Republicans would need 51 votes to dismiss the case, and Democrats would need 51 votes to call witnesses. Republicans have a 53-47 majority in the Senate.