Sen. Lamar Alexander (R-TN) announced Thursday night that he will vote against calling additional witnesses in the Senate impeachment trial of President Donald Trump.

“I worked with other senators to make sure that we have the right to ask for more documents and witnesses, but there is no need for more evidence to prove something that has already been proven and that does not meet the United States Constitution’s high bar for an impeachable offense,” Alexander said in a statement.

“There is no need for more evidence to conclude that the president withheld United States aid, at least in part, to pressure Ukraine to investigate the Bidens; the House managers have proved this with what they call a ‘mountain of overwhelming evidence,’ the senator added. “There is no need to consider further the frivolous second article of impeachment that would remove the president for asserting his constitutional prerogative to protect confidential conversations with his close advisers.”

The retiring Tennessee Republican’s decision signals that the Senate will not have the support required to subpoena more witnesses and evidence at Friday’s highly anticipated vote and all but points to the trial coming to a swift close. 51 Senate votes are needed to impel witnesses or 50 if Supreme Court Chief Justice John Roberts were to cast the unlikely tie-breaking vote.

The development comes amid calls from Democrats to hear from former White House National Security Advisor John Bolton. On Sunday, the New York Times reported his forthcoming book alleges that President Trump sought to link U.S. military aid to Ukraine to investigations into allegations of corruption against former Vice President and 2020 Democrat candidate Joe Biden, and his son, Hunter Biden. President Trump has denied the allegations, calling them categorically “false” while meeting with Israeli Prime Minister Benjamin Netanyahu in the Oval Office Wednesday. Earlier Wednesday, President Trump slammed Bolton in a pair of tweets, accusing him of writing a “nasty & untrue book” about his tenure in the White House.

Before Thursday’s Senate question period adjourned, Alexander and fellow moderate Republican Sen. Lisa Murkowski (AK) joined a question asking that even if former White House National Security Advisor John Bolton was to testify, “isn’t it true that the allegations would not rise to an impeachable offense” and “his testimony would add nothing?”

Breaking with the overwhelming majority of her Republican colleagues was Sen. Susan Collins (ME), who announced that she will vote for witnesses as “the most sensible way to proceed.”

“I believe hearing from certain witnesses would give each side the opportunity to more fully and fairly make their case, resolve any ambiguities, and provide additional clarity. Therefore, I will vote in support of the motion to allow witnesses and documents to be subpoenaed,” the Maine Republican said in a statement.

I will vote in support of the motion to allow witnesses and documents to be subpoenaed. My full statement: https://t.co/VuhZv6CO5e pic.twitter.com/LhQlnvPaoc — Sen. Susan Collins (@SenatorCollins) January 31, 2020

“If this motion passes, I believe that the most sensible way to proceed would be for the House Managers and the President’s attorneys to attempt to agree on a limited and equal number of witnesses for each side. If they can’t agree, then the Senate could choose the number of witnesses,” she added.