President Donald Trump’s impeachment trial faces a climactic vote on Friday, when senators are due to decide whether to call witnesses and prolong the historic proceeding or instead bring them to the swift conclusion that Trump wants.
Senate Democrats have been arguing throughout the two-week trial that lawmakers need to hear from witnesses like John Bolton, Trump's former national security adviser. But they do not appear to have enough support from Republicans who control the chamber.
Republican Senator Lamar Alexander, who had been undecided, declared late on Thursday that further evidence in the case was unnecessary.
Barring an unforeseen change of heart by another Republican senator, that would leave Democrats short of the 51 votes they need and allow Trump's allies to defeat the request for additional evidence and move towards a final vote that is all but certain to acquit the president.
That final vote could take place late on Friday or on Saturday, congressional sources said.
"The truth is staring us in the eyes," said Democratic Representative Adam Schiff, the lead House prosecutor, said on the Senate floor.
"We know why they don't want John Bolton to testify. It's not that we don't really know what's happened here. They just don't want the American people to hear it in all of its ugly, graphic detail."
The Democratic-controlled House of Representatives impeached Trump in December, formally accusing him of abusing his power for pressuring Ukraine to investigate a political rival, Democratic presidential candidate Joe Biden. The House also charged Trump with obstruction of Congress.
Trump is only the third president in U.S. history to be impeached. A vote of two-thirds of the Senate is required to remove him from office and no Republicans have indicated they will vote to convict.
Trump's Republican allies have tried to keep the trial on a fast track and minimize any damage to the president, who is running for re-election.
Trump's acquittal would allow him to claim vindication just as the Democratic Party holds its first nominating contest for the Nov. 3 election in Iowa on Monday.
He held a rally in Des Moines, Iowa, on Thursday night and denounced the trial, calling it an effort by Democrats to overthrow his 2016 election victory.
"They want to nullify your ballots, poison our democracy and overthrow the entire system of government,” Trump said.
On Friday, each side is expected to present closing arguments before the Senate votes on whether to call witnesses.
Possible testimony from Bolton is of particular interest after a report - which he has not denied - that he planned to say in an upcoming book that Trump told him he wanted to freeze $391 million in U.S. military aid for Ukraine until it investigated Biden and his son, Hunter.
If further witnesses and documents are permitted, Republicans have threatened to call either Joe or Hunter Biden and perhaps the whistleblower within the government whose complaint about Ukraine led the House to begin its investigation.
If the vote on whether to allow witnesses is 50-50, Chief Justice John Roberts could step in to break the tie. But there is so little precedent for impeachment trials that Senate aides said there was no way to know exactly what would occur.
If Roberts declines to break a tie, the deadlock would mean a defeat for Democrats.