Four GOP senators would need to break with their party on a resolution seeking testimony.
The House managers haveare finishing up their opening arguments in their case against President Donald Trump — but it's still unclear whether they'll be able to present any new evidence.
"Every day more and more of the public is watching," Senate Minority Leader Chuck Schumer said Thursday. "I am more hopeful than ever that four conscientious brave Republicans will come forward and tell (Senate Majority Leader) Mitch McConnell you can't shut this down without witnesses, you can't shut this down without documents."
With the GOP holding a 53-47 majority in the Senate, Democrats would need at least four Republicans to cross party lines in order to be able to call witnesses or subpoena documents in the third presidential impeachment trial in American history.
Here's a look at some of the Senate Republicans who could cross party lines and where they stand.
Mitt Romney, Utah
Romney, who is Trump's most outspoken Republican critic in the Senate, has he'd "love to hear" what Bolton has to say. He's declined comment on his thoughts during the trial.
"I won't speak to the process until the entire thing is done," he said Thursday.
Susan Collins, Maine
Collins, who is up for re-election later this year, said before the trial senators "should be completely open to calling witnesses."
She also said she was working with a "fairly small group of Republicans" to make sure there was a procedure in the impeachment trial rules allowing senators to call for witnesses after opening arguments and senators get to question both sides. Collins hasn't said anything tipping her hand on the witnesses issue since the Democrats made their case.
Lisa Murkowski, Alaska
Murkowski said in an interview on Christmas Eve that she was "disturbed" that McConnell would engage in "total coordination" with the White House regarding the trial.
She, along with Romney and Collins, refused in October to add her name to a resolution condemning the House impeachment inquiry. But she's also been critical of the House impeachment inquiry, and said she was "offended" by the House impeachment managers accusation that senators would be involved in a cover-up if they did not support new evidence or witnesses at trial, a Murkowski aide told NBC News.
Murkowski told reporters on Saturdayshe wanted to "really hear the case" and ask questions "before we make that determination as to, what more do we need. I don't know what more we need until I've been given the base case." She'll have the chance to pose questions next week.
Lamar Alexander, Tennessee
Alexander said Monday he'd vote against a motion to dismiss the case if one were presented because he was weighing whether to hear from additional witness.
"I think we need to hear the case; ask your questions," Alexander, who is retiring at the end of his termand is close with McConnell,told NBC News. "Then, as they did in the Clinton impeachment, we ought to decide then whether we need to hear from additional witnesses or need additional documents."
He has declined to say where he stands on witnesses since the Democrats' arguments began this week.
Others Republicans to watch...
Sen. Cory Gardner of Colorado is up for re-election and considered perhaps the most vulnerable GOP senator in 2020, making his approach on impeachment difficult to read. He repeatedly criticized the House investigation.
Sen. Shelley Moore Capito of West Virginia has also been noncommittal, saying the Senate should decide on whether Bolton should testify later in the trial.
The vote on whether to hear witnesses is expected to be held late next week.