While Alexander announced he would not back more witnesses, Sen. Susan Collins (R-ME) said she would, joining Sen. Mitt Romney (R-UT) as being in favor of extra evidence in the President's trial.
"I will vote in support of the motion to allow witnesses and documents to be subpoenaed," Collins said in a written statement issued shortly before the Alexander announcement.
It was still possible that a vote on witnesses could end in a 50-50 tie, if Sen. Lisa Murkowski (R-AK) joins Romney, Collins, and all 47 Democrats in voting for witnesses, that would produce a deadlocked Senate.
Murkowski said she would announce her decision on Friday.
If the witness vote ends in a 50-50 tie, it would produce a unique parliamentary situation in the Trump impeachment trial.
Normally, the Vice President would have the only power to break a tie vote in the Senate - but in cases of Presidential impeachment, the job of presiding over the Senate falls to the Chief Justice - and there is precedent from 1868 that a Chief Justice can break ties on procedural matters.
If Chief Justice Roberts refused to vote, the witness effort would fail on a tie vote.
On Thursday, there was little suspense about the eight hours of questions by Senators for lawyers on both sides, as House prosecutors and the White House legal team used the time to make their very familiar points.
"If we have a trial with no witnesses, that will be a new precedent," said Rep. Adam Schiff (D-CA), who answered the majority of questions for the House managers.
"There are some in this room that are days away from the Iowa Caucuses taking place," said Jay Sekulow, the President's personal attorney, referring to the Democratic Senators running for President, as the White House legal team argued Mr. Trump was the victim of a political vendetta.
Only a handful of questions caused reporters to sit up and take notice, like one from Sen. Lisa Murkowski (R-AK), which directly asked the President's team about testimony from former National Security Adviser John Bolton.
Addressing concerns voiced by the White House legal team that extra witnesses would bog down the Senate for weeks or months, Schiff proposed on Thursday a one week delay in the trial, to give time for depositions.
"The proposal is we take a week, the Senate goes about its business, we do depositions," Schiff said.
But like other ideas floated on witnesses in the past week, there was no indication that it would draw bipartisan support.
And by midnight on Thursday night, Republicans felt more and more confident that by some time late on Friday night, the Senate would vote to reject the impeachment charges against President Trump.
"Justice will be done. President Trump will be ACQUITTED," tweeted Rep. Mark Meadows (R-NC), a strong Trump ally in the House.