Sen. Lamar Alexander (R-Tenn.) said Thursday he’ll vote against the Senate calling witnesses in President Donald Trump’s impeachment trial, all but eliminating the prospect of individuals like former national security adviser John Bolton testifying and making acquittal almost certain.

Alexander, who is retiring from the Senate after this term, said in a Thursday evening statement that “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.”

“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,” he continued, adding that voters should decide whether Trump remains in office.

Four Republicans would need to side with Senate Democrats to allow more witnesses. With Alexander out, it’s extremely likely Republicans will have the votes to reject witnesses and move to conclude the trial.

Sen. Susan Collins (R-Maine) announced late Thursday she will vote to hear from witnesses. Two other Republican senators — Lisa Murkowski (Alaska) and Mitt Romney (Utah) — have strongly hinted they would vote in favor of allowing witnesses. Murkowski told reporters she would announce her decision on Friday morning. If those three do side with Democrats and the vote results in a tie, Supreme Court Chief Justice John Roberts, who is presiding over the trial, could be called in to break the tie, although most legal experts agree he’s unlikely to weigh in.

Senate Majority Leader Mitch McConnell (R-Ky.) has opposed extending the trial to allow new evidence from additional witnesses, arguing that House investigators should have conducted the necessary subpoenas and that allowing further testimony would unnecessarily drag out the proceedings.

Most Republicans have followed his lead — including some viewed as potential swing voters, such as Sen. Cory Gardner (R-Colo.). Gardner, who is up for reelection in the fall, announced his decision Wednesday.

Despite McConnell’s opposition, Democrats have pressed for witness testimony. Their efforts have focused on Bolton, who declined to participate during the House of Representatives’ impeachment proceedings last year. However, revelations from a leaked draft of Bolton’s forthcoming book renewed scrutiny on the former Trump administration official, particularly his claim that Trump tied military aid for Ukraine to the country announcing it would investigate former Vice President Joe Biden and his son Hunter.

“We have a witness with firsthand evidence of the president’s actions for which he is on trial. He is ready and willing to testify. How can Senate Republicans not vote to call that witness and request his documents?” Senate Minority Leader Chuck Schumer (D-N.Y.) said in a Monday news conference. “We’re all staring a White House cover-up in the face.”

Although Alexander said he did not need to hear from witnesses, he suggested he agrees with the impeachment managers that Trump pressured the Ukrainian government to investigate a political rival ― he just doesn’t want to hear from more witnesses about it.

Alexander said there was “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.”

“There is no need for more evidence to prove that the president asked Ukraine to investigate Joe Biden 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. “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.’”

He was more firmly behind Trump in terms of the allegation that he obstructed justice, calling it a “frivolous second article of impeachment that would remove the president for asserting his constitutional prerogative to protect confidential conversations with his close advisers.”