Updated 11:28 p.m. ET
Sen. Lamar Alexander said on Thursday night that he will not vote to allow witnesses and evidence into the impeachment trial of President Trump.
"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,'" Alexander said in a statement. "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."
The decision delivers a devastating blow to House managers prosecuting Trump, all but ensuring witnesses will not be called for Trump's trial.
Without sufficient support for bringing witnesses, Senate Majority Leader Mitch McConnell is expected to move quickly to vote on the two articles of impeachment. The president could be acquitted on both charges as early as Friday evening.
Democrats needed four Republicans to join them in order to have enough votes to call witnesses to testify. Alexander, R-Tenn., had been largely silent on the issue and was considered a crucial swing voter. Only two Republicans — Maine Sen. Susan Collins and Utah Sen. Mitt Romney — have publicly called for witness testimony.
Collins said late Thursday she would support bringing witnesses.
Sen. Lisa Murkowski indicated she would share her decision on witnesses on Friday morning after spending some time studying her notes from the trial. "I'm going to go back to my office, I'm going to put some eye drops so that I can keep reading, and I've been forming a lot of thoughts and so that's going to be my job now at almost 11 o'clock," she said.
But despite Alexander's opposition to witnesses, he blasted President Trump's actions in a statement announcing his vote. "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," he said. "When elected officials inappropriately interfere with such investigations, it undermines the principle of equal justice under the law. But the Constitution does not give the Senate the power to remove the president from office and ban him from this year's ballot simply for actions that are inappropriate."
For days, the Democratic prosecutors have been making the case that Trump's trial would not be fair in the absence of evidence and witnesses. Trump's legal team has countered that opening the door to witnesses could prolong the trial for weeks, or even months. Trump's lawyers have said that nothing any witness could testify to would create the basis to convict and remove Trump from office.
Earlier on Thursday, Senate Minority Leader Chuck Schumer said a trial without witnesses would "render the president's acquittal meaningless," because, Schumer said, the process would look "so rigged in his favor."
Alexander's announcement to vote in lockstep with Republican leadership makes it increasingly doubtful that senators will ever hear from former national security adviser John Bolton. He was a highly anticipated witnesses since President Trump told Bolton he wanted to keep the hold on Ukrainian military aid until officials there agreed to investigate Democrats, including the Biden family.
Prosecutors say his firsthand account demonstrates the abuse of power allegations at the heart of the impeachment charges. Pressure on Republicans to allow Bolton to speak during the trial began to mount following media reports about Bolton's yet-to-be-published book, which supposedly contains dozens of pages dedicated to Trump's affairs with Ukraine.
Before Alexander's announcement, Trump's lawyers made the case for why they do not view Bolton's testimony as relevant to the proceeding.
"The idea that something comes out because someone makes an assertion in a book allegedly. It's simply alleged now that the manuscript says that," said White House lawyer Patrick Philbin, "That then we should start having this chamber call new witnesses and establish a new normal for impeachment proceedings as being that there doesn't have to be a complete investigation in the House. I think that is very damaging for the future of this institution."
Collins, announcing her plans to vote for witnesses, wrote: "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."
Barring the appearance of a Republican wildcard, Democrats may fall one vote short of being able to call witnesses.
With a 53-47 majority in the Senate, Republicans are expected to vote to acquit Trump as early as Friday.
Removing Trump from office would take a two-thirds majority.