Updated at 1:43 p.m. ET

Sen. Lamar Alexander, R-Tenn., says that while he believes President Trump has acted inappropriately, he does not need to hear from witnesses in Trump’s impeachment trial.

Alexander is among a key group of Republicans whom Democrats hoped to persuade to join them in their effort to call witnesses.

“I don’t need to hear any more evidence to decide that the president did what he’s charged with doing,” Alexander told NPR’s Steve Inskeep on Friday. “So if you’ve got eight witnesses saying that you left the scene of an accident, you don’t need nine.”

Alexander announced on Thursday night that he would not vote to call witnesses, all but assuring the trial will end without senators hearing additional testimony.

“I agree he did something inappropriate, but I don’t agree he did anything akin to treason, bribery, high crimes and misdemeanors. I think there’s a big gap there,” Alexander told NPR.

Citing an argument by Trump attorney Alan Dershowitz, Alexander said: “Forty presidents have been accused of abuse of power since Washington. So, many presidents have done inappropriate things. I think it’s appropriate for me as a United States senator to say, ‘Mr. President, you shouldn’t do that.’ But that doesn’t mean that I should take it upon myself to vote, to remove him from office and take him off the ballot. I think that’s up to the people.”

Alexander, who has announced he will be retiring at the end of his current term, said he still supports President Trump’s reelection.

It would take four GOP senators voting with the Democratic caucus to call for witness testimony.

Shortly after Alexander made his announcement on Thursday, Sen. Susan Collins, R-Maine, said she will vote in favor of calling witnesses.

In a statement, she said, “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.”

As the impeachment trial resumed Friday, Sen. Lisa Murkowski, R-Alaska, said she will vote no on witnesses. Murkowski is another Republican whom Democrats had hoped would support hearing from witnesses.

President Trump’s impeachment trial could end in acquittal as soon as Friday evening, following the announcement from a crucial Republican senator that he would not be supporting witnesses.

Sen. Lamar Alexander, R-Tenn., said late Thursday that he will not join Democrats in their push to subpoena witnesses. The news dashed House managers’ hopes of beginning a new phase of the trial, potentially airing testimony from witnesses who could have proved politically damaging to Trump.

The looming acquittal for the president was long anticipated, but it still marks a striking setback for Democrats, who at times appeared hopeful that they would be able to persuade enough Republicans to join them in voting to call witnesses, thereby prolonging the trial if not the altering its outcome.

The president faces two articles of impeachment: abuse of power and obstruction of Congress related to accusations that he held up military aid to Ukraine until the country announced investigations into potential political rival former Vice President Joe Biden and his son Hunter.

Former national security adviser John Bolton has been mentioned as a witness after reports that he can verify those accusations.

Senators on Friday, starting at 1 p.m., will hold four hours of debate equally divided between House managers prosecuting the case and Trump’s defense lawyers.

Afterward, a vote will be held on a motion to consider evidence or witness testimony. The Democratic caucus needs four Republicans to defy their party in order to succeed, and Alexander’s decision to vote with his party all but guarantees that the witnesses will not be part of the trial.

Sen. Susan Collins, R-Maine, on Thursday announced she plans to for vote for witnesses, writing: “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.”

A spokeswoman for Sen. Mitt Romney, R-Utah, said on Twitter that Romney “wants to hear from Ambassador Bolton, and he will vote in favor of the motion today to consider witnesses.”

The one other moderate Republican who signaled she may be open to voting for witnesses is Lisa Murkowski of Alaska. With 53 Republicans, 45 Democrats and two independents who caucus with Democrats, there is the prospect of a 50-50 vote on whether to call witnesses. Such a vote would fail, as the presiding officer, U.S. Chief Justice John Roberts, is seen as unlikely to insert himself into what is essentially a political process.

On Thursday, after two days of questions from senators, which included forays into foreign election interference and broad interpretations of executive power, both sides dug deep in their respective positions. Democrats argued that Trump solicited the help of a foreign country in order to tilt this year’s presidential election in his favor and that he should therefore be removed from office. Yet Trump’s defense team said the prosecution was a politically driven effort to reverse the result of the 2016 election.

“Now it’s up to the Senate to decide what the facts are,” Alan Dershowitz, one of the president’s impeachment lawyers, told NPR’s Steve Inskeep on Friday. “But my position was very clearly that if a president is charged with abuse of power or obstruction of Congress, that the charges should be dismissed. They are not within the constitutional criteria.”

The focus on Bolton was triggered by media reports describing portions of his forthcoming book in which he purportedly wrote that he had a conversation with Trump in which the president said the release of security assistance to Ukraine would be contingent on the country announcing investigations into the Bidens.

Trump, speaking at a campaign rally in Des Moines, Iowa, on Thursday night, attacked Democrats for charging him with high crimes and misdemeanors.

“Can you believe these people? I got impeached. They impeached Trump,” said Trump. “They want to nullify your ballots, poison our democracy and overthrow the entire system of government.”

Democrats, who have maintained throughout the trial that a proceeding without witnesses and evidence would not be a fair process, sought Thursday to undercut the president’s likely imminent acquittal.

Senate Minority Leader Chuck Schumer, D-N.Y., said a trial “without the evidence, without witnesses and documents would render the president’s acquittal meaningless,” adding Trump’s impeachment trial will have a “giant asterisk next to it, because the trial was so rigged in his favor.”