GOP Sen. Susan Collins said she's working with a "fairly small group" of fellow Republicans to ensure witnesses are called to testify during President Donald Trump's Senate impeachment trial.

"We should be completely open to calling witnesses," Collins said on Friday, according to Maine's Bangor Daily News.

John Bolton, President Donald Trump's former national security adviser, put pressure on GOP lawmakers when he announced earlier this week that he's willing to testify.

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Republican Sen. Susan Collins revealed that she's working with a "fairly small group" of fellow GOP senators to ensure witnesses are called to testify during President Donald Trump's Senate impeachment trial.

"We should be completely open to calling witnesses," Collins said on Friday, according to Maine's Bangor Daily News. "I am 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."

A simple majority — 51 votes — is required to set the trial's rules, so Collins needs to convince three other Republicans to join her and the 47 Senate Democrats in forcing witnesses to the stand. Senate Majority Leader Mitch McConnell has pledged to work "in total coordination" with Trump and the White House counsel's office.

Collins and other vulnerable Republican senators supported McConnell's decision to move forward with a trial without guaranteeing that the Senate will call witnesses. A moderate from Maine, Collins is running in a competitive race for reelection this year.

John Bolton, President Donald Trump's former national security adviser, put pressure on GOP lawmakers when he announced earlier this week that he's willing to testify in the impending Senate impeachment trial, even if the president opposes it.

GOP Sen. Mitt Romney said this week that he'd "love to hear" from Bolton during the trial.

With Bolton, Democrats now have a key witness — someone with direct knowledge of Trump's dealings with Ukraine — ready and willing to make what could be very damaging claims against the president. Many Republicans appear willing to deny Bolton the chance to divulge information about what his lawyer has said are "many relevant meetings and conversations."

Shortly after Bolton's announcement, McConnell declared the Senate would decide on whether to call witnesses once the trial is underway, similar to President Bill Clinton's impeachment proceedings in 1998.