GOP Sen. Lisa Murkowski said Friday she would not vote for more witnesses in President Trump’s impeachment trial, thwarting Democrats’ efforts to have more evidence introduced in the proceeding.

Democrats needed to persuade four Republicans to vote with them in the Senate in order to call witnesses such as John Bolton, Trump’s former national security adviser, and secure documents the White House has withheld.

Sen. Mitt Romney of Utah on Friday became the second Republican in the upper chamber to back voting for witnesses, joining Susan Collins of Maine.

But GOP Sen. Lamar Alexander of Tennessee, who had purportedly been on the fence, said late Thursday that Democrats had proven the case against Trump but that the president’s actions did “not meet the United States Constitution’s high bar for an impeachable offense.”

He therefore would be voting against calling witnesses, meaning that Murkowski’s decision will likely result in a 51-49 vote against new witnesses.

Technically, Supreme Court Chief Justice John Roberts would be a tie-breaker, as he presides over the Senate instead of the vice president, in the case of impeachment, but many observers believed he would follow the lead of the late William Rehnquist, who during the Clinton impeachment, in his own words, proudly did “nothing in particular” and left it to the senators’ votes.

A tie on the motion to call witnesses or subpoena further evidence would be a defeat for the Democrats’ motion, as a simple majority of 51 is needed to pass it.

The chamber would then move toward a final vote that is all but certain to acquit the president, as a two-thirds majority is needed to convict, requiring some 20 Republicans to defect. That final vote could take place late Friday or possibly Saturday.

This would be the first Senate impeachment trial in US history with no witnesses, including trials of two prior presidents and a number of other federal officials.

The Democrat-controlled House impeached Trump in December, formally charging him with abuse of power for asking Ukraine President Volodymyr Zelensky to investigate a political rival, Democratic presidential candidate Joe Biden, at a time when Trump was also withholding congressionally approved security aid for Ukraine. The president has denied any linkage between the two.

The House also charged Trump with obstruction of Congress for blocking current and former officials from providing testimony or documents.

On Friday, the Democrats prosecuting Trump and the president’s lawyers are expected to present closing arguments of up to two hours each that can also include arguments about whether to call witnesses.

Then the Senate will vote on the witnesses issue first. A vote on each of the two impeachment articles could follow.

With wires