The lawmakers are also expected to discuss the issue of the president potentially using executive privilege to block testimony.
ADVERTISEMENTSenate Republicans will meet Tuesday afternoon after President Donald Trump'simpeachment trial adjourns— and the question of whether to call witnesses in the trial is expected to be discussed, four GOP aides told NBC News.
The meeting of the Senate Republican conference will be held for "starting to check the conference on witnesses," a GOP leadership said. At a Senate Republican lunch ahead of the meeting executive privilege will likely be discussed.
Conversations about where the Senate Republicans are on the witness question have been ongoing. But with Trump defense team's arguments set to conclude Tuesday afternoon, those discussions have taken on a new sense of urgency.
A debate and vote on whether to call witnesses could come later this week.
Republicans have a 53-47 majority in the Senate, meaning Democrats would need four Republicans to join them in a vote for witness testimony in the Senate trial.
Top Senate Democrats have said repeatedly they want former national securityadviser John Bolton, acting White House chief of staff Mick Mulvaney, Robert Blair, senior adviser to Mulvaney, and Michael Duffey, associate director for national security at the Office of Management and Budget to testify.
However, calls for Bolton, in particular, to testify have intensified in recent days after The New York Times reported — according to a manuscript of Bolton's book, which it obtained and has not seen by NBC News — that Trump told Bolton in August that nearly $400 million in aid to Ukraine would not be released until it provided all of the information it had in connection with the investigations of Democrats the president sought.
A pair of moderate Republican senators — Mitt Romney, of Utah, and Susan Collins, of Maine — said Monday that the report of major revelations in Bolton's soon-to-be-released book strengthens the case for calling witnesses.
Romney, Collins, Lisa Murkowski of Alaska and Lamar Alexander of Tennessee are considered to be the most likely Republicans to vote for witnesses.
Murkowski said Monday: "I've said before I'm curious about what Ambassador Bolton might have to say. I'm still curious." Alexander said he won't decide until after both sides have answered questions from the Senate.
Sen. Lindsey Graham. R-S.C., a top Trump ally who's resisted calls for additional witnesses and documents, acknowledged Monday that Bolton may be "a relevant witness" and said he'd consider subpoenaing a manuscript of his book.