Cliffhanger: What you need to know about witnesses and Trump's trial

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By Kasie Hunt and Allan Smith and Frank Thorp V  with NBC News Politics
Image: Mitt Romney
Senator Mitt Romney walks outside the Senate chamber during a recess in the impeachment trial against President Donald Trump at the US Capitol on Jan. 8, 2020.   -   Copyright  Brendan Smialowski AFP - Getty Images file

Ahead of the vote on Friday afternoon on whether to call witnesses at President Donald Trump's impeachment trial, GOP Senate leaders believe they will have just enough votes to block additional testimony and documents.

In order for additional witness testimony to be approved, four Republicans in the Senate would need to vote alongside all Democrats.

So far, only Sen. Mitt Romney, R-Utah, has indicated he will vote in favor of witnesses, and Sen. Susan Collins, R-Maine., has said it is likely she will, too. Sen. Lisa Murkowski, R-Alaska, has hinted at an interest in hearing from witnesses but has not provided a strong indication of how she will vote.

Other Republicans who were Democratic targets have recently said they will not be voting with the Democrats. Those senators include Cory Gardner, R-Colo., Pat Roberts, R-Kan., and Steve Daines, R-Mont. Meanwhile, Sen. Pat Toomey, R-Pa., said it is "very, very unlikely" he will vote for witnesses. Additionally, Sen. Jerry Moran, R-Kan., is unlikely to do so.

There are a couple of senators who are leaning no at the moment. They are Sens. Lamar Alexander, R-Tenn., and Rob Portman, R-Ohio. If Romney, Collins and Murkowski voted for witnesses, either Alexander or Portman would be crucial to Democrats in breaking a tie and getting the 51 votes needed for additional testimony.

In the event of a 50-50 tie, the vote on witnesses is expected to fail as Chief Justice John Roberts, who is presiding over the trial, is highly unlikely to weigh in.

Speaking with "Fox & Friends" on Thursday, Sen. Mike Braun, R-Ind., addressed the possibility of a tie vote.

"That's tricky," Braun said, "because there there's a question of how much sway the presiding officer would have over that dynamic."

Braun said Roberts "could" decide to break the tie.

"And then if 51 Senators don't like the decision, you could overrule the presiding officer," he said. "So we are getting into some territory that's probably uncharted here."

Democrats have demanded the Senate call a series of witnesses, including Trump's former national security adviser John Bolton and acting chief of staff Mick Mulvaney. This week, The New York Times reported that, according to a manuscript of Bolton's upcoming book described to the publication, Bolton alleges Trump directly linked Ukrainian aid and the investigations he sought in an August conversation with his former top aide. Trump has denied having such a discussion.

NBC News has not seen the manuscript or verified the claim.

On the other hand, Republicans have pledged to call for testimony from people like former Vice President Joe Biden's son Hunter Biden and the whistleblower, among others, should witnesses be approved.

Every impeachment trial in history has had witness testimony. Twenty-six witnesses testified in the 2010 trial of Judge Thomas Porteous, the most recent impeachment trial, including 17 who did not testify before the House.