Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderMcConnell locks down key GOP votes in Supreme Court fight Alexander backs vote on Trump Supreme Court nominee: What Democrats 'would do if the shoe were on the other foot' Toobin: McConnell engaging in 'greatest act of hypocrisy in American political history' with Ginsburg replacement vote MORE (R-Tenn.) announced Thursday night that he will vote against a motion to consider subpoenas for additional witnesses and documents at the impeachment trial, putting the chamber on track to acquit President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE on Friday or Saturday.

“There is no need for more evidence to prove that the president asked Ukraine to investigate Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE and his son, Hunter; he said this on television on October 3, 2019, and during his July 25, 2019, telephone call with the president of Ukraine," Alexander said in a statement released shortly after the Senate ended 16 hours of questions to the impeachment managers and lawyers for Trump's defense.

"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.’ 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," his statement said.

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Alexander, however, admonished Trump for “inappropriate” conduct.

“It was inappropriate for the president to ask a foreign leader to investigate his political opponent and to withhold United States aid to encourage that investigation. When elected officials inappropriately interfere with such investigations, it undermines the principle of equal justice under the law,” he warned.

But the senior lawmaker argued that the Constitution does not give the Senate the power to remove the president from office and ban him from the ballot merely for inappropriate actions.

“Our founding documents provide for duly elected presidents who serve with ‘the consent of the governed,’ not at the pleasure of the United States Congress. Let the people decide,” he said.

Alexander's decision makes it significantly less likely that Democrats will have enough votes to call former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE and other witnesses to testify, which would have extended the trial past the weekend and perhaps much longer.

With Alexander joining the overwhelming majority of the Senate GOP conference, it now appears there are no more than three Republican votes for witnesses, which means the question would deadlock in a 50-50 tie or fail 49-51.

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Sen. Susan Collins (R-Maine) announced Thursday evening she would vote to subpoena witnesses and documents and Sen. Mitt Romney (R-Utah) is expected to announce the same decision Friday.

“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,” Collins said in a statement.

Sen. Lisa Murkowski (R-Alaska) remains the sole undecided vote on the crucial procedural question. She told reporters Thursday that she would announce her position Friday.

“I got two — you can actually take pictures of my two volumes here,” Murkowski told a crowd of reporters, pointing to her voluminous notes from the trial. “I’m going to go back to my office [and] put some eye drops in so I can keep reading. And I’ve been forming a lot of thoughts.”

Republicans control 53 seats and at this point the best Democrats can hope for is a 50-50 tie.

Chief Justice John Roberts conceivably could step in to break a tie vote, but lawmakers on both sides of the aisle say that’s unlikely because it could politicize his role.

The Senate is set to debate and vote on the motion to consider additional evidence on Friday. If it fails, as expected, Republican leaders plan to move as quickly as possible to a final up-or-down vote on the articles of impeachment.

Alexander, a longtime close friend of Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.), for days had refused to tell reporters and colleagues how he would vote on the crucial question of calling new witnesses and documents for Trump’s trial.

The senior senator from Tennessee emerged unexpectedly as a possible Republican defector earlier this month after he joined Romney, Collins and Murkowski to add language to the organizing resolution guaranteeing a vote on considering additional subpoenas after opening arguments.

Alexander has announced he will retire from the Senate at the end of the year after three terms in Congress, which observers thought would make him less susceptible to pressure from Trump and his allies than colleagues facing future primary races.

He and Murkowski had been the two undecided votes in the GOP conference after Romney and Collins announced at the start of the trial that they would likely vote for witnesses.

Alexander and Murkowski had been spotted sitting together in recent days at Senate Republican dinner meetings, a tactic they appeared to adopt to shield themselves somewhat from peer pressure from colleagues eager to persuade them.

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Speculation about Republican moderates bucking McConnell on the witness question picked up considerable momentum after The New York Times reported Sunday that Bolton had circulated a book manuscript claiming that Trump explicitly linked military aid for Ukraine to officials there announcing an investigation of former Vice President Joe Biden.

During the days of uncertainty, however, GOP aides predicted that Alexander would not pull the rug out from McConnell, one of his closest colleagues in the Senate.

He and McConnell met in 1969 as young men starting their careers in Washington, when Alexander was working in the Nixon White House and McConnell was a staffer on Capitol Hill.

Updated at 11:46 p.m.