The odds of the Senate calling witnesses in its impeachment trial are diminishing, after Sen. Lamar Alexander (R-Tennessee) announced Thursday he will vote against the motion.

“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.

The retiring senator was viewed as a key swing vote, and previously told reporters he “maybe” will vote for witnesses or “maybe not.”

He believes there is no question about the president’s actions in Ukraine — he withheld U.S. aid and an Oval Office meeting with the foreign country’s leader in an attempt to pressure the government to launch an investigation into Democratic nominee frontrunner, Joe Biden.

But Alexander thinks it should be up to the American people to judge his actions.

“The question then is not whether the president did it, but whether the United States Senate or the American people should decide what to do about what he did,” Alexander said. “I believe that the Constitution provides that the people should make that decision in the presidential election that begins in Iowa on Monday.”

Two GOP senators, Susan Collins (R-Maine) and outspoken Trump critic Mitt Romney (R-Utah), will side with the 47 Democratic Caucus members in supporting calling witness testimony.

Romney’s communications director, Liz Johnson, confirmed in a tweet that the senator “wants to hear from Ambassador [John] Bolton, and he will vote in favor of the motion today to consider witnesses.”

Collins also emphasized the need to hear from Bolton, whose unpublished manuscript of an upcoming book claimed Trump had directly told him he wanted to withhold aid to Ukraine until they agreed to publicly announce an investigation into the Biden family.

“I’ve always said that I was likely to call witnesses, just as I did in the 1999 Clinton trial,” she said. “The reports about John Bolton’s book strengthen the case for witnesses and have prompted a number of conversations among my colleagues.”

The two GOP senators’ defections leave the Democrats with 49 votes, leaving all eyes on Sen. Lisa Murkowski (R-Alaska) who will likely decide whether the vote ends in a tie or there is a majority against calling witnesses.

Murkowski indicated Thursday she may support calling witnesses, as she asked, “Why should this body not call Ambassador Bolton?”

If the vote ends in a tie, Chief Justice John Roberts may become the historical tiebreaker.