BBC.COM- Image copyright Getty Images Image caption Senator Susan Collins is among four wavering Republicans
All eyes were on four Republican senators ahead of Friday to see if they would vote to call witnesses to Donald Trump’s impeachment trial.
If four moderate Republicans decide to vote with Democrats, it is likely the trial will hear from the former National Security Adviser, John Bolton.
Senior Republicans had pushed for a speedy trial with no witnesses or new evidence presented to senators.
But reports that Mr Bolton may have damning testimony shifted the calculus.
Senators sat in session on Thursday for a second day of questions to Democratic impeachment managers and Trump defence lawyers. The senators are expected to vote on witnesses on Friday.
Why are the four so important?
The four moderate Republicans – Senators Mitt Romney, Lisa Murkowski, Susan Collins, and Lamar Alexander – have all indicated they remain undecided on how to vote.
All four would be required to vote with a unanimous Democratic block in order to achieve the necessary Senate majority.
Senator Alexander appeared poised on Thursday to announce his decision, telling the Washington Post that he would make his mind up after the final questions in the Senate chamber.
According to Huffington Post reporter Ryan Reilly, Mr Alexander had a Senate page deliver a note to Senator Mitch McConnell – the most senior Republican in the Senate, who has led the charge for a speedy acquittal of President Trump.
“McConnell let it sit on his lap for a couple minutes, then read it and stuffed it in his suit jacket pocket. McConnell’s face is tough to read. But he didn’t look in Alexander’s direction,” Mr Reilly wrote on Twitter.
If Democrats lose the vote, and no witnesses are called, the president could be acquitted of the impeachment charges as soon as Friday. A two-thirds majority in the Senate is required to remove him from office, and Republicans hold a 53-47 majority in the Senate.
If all four Republicans decide to vote with Democrats, the trial will go on into next week. The president would remain highly likely to be acquitted.
Why is Bolton so important?
A bombshell report in the New York Times earlier this week said that Mr Bolton had written in his upcoming book that the president told him directly that military aid was being withheld from Ukraine in exchange for dirt on a Democratic political rival – the key impeachment charge against the president.
The report reinvigorated Democrats’ attempts to call new witnesses to the trial, and simultaneously energised Republican efforts to push the process through without anyone being called.
If the reports about Mr Bolton were true, and he testified to that effect, he would be the first witness in the process to directly link the president to an alleged quid pro quo with Ukraine and an abuse of presidential power.
Mr Trump’s lawyer expanded their defence in the Senate earlier this week to suggest that anything a president does in service of his own re-election, believing that to be in the public interest, cannot be impeachable. The tactic shocked Republicans and Democrats alike.
How did the White House react?
The White House pushed back against the publication of Mr Bolton’s book, citing security concerns. The National Security Council alleged that the book had “top secret” details that must be removed, a claim Mr Bolton rejects.
Mr Bolton’s lawyer Charles Cooper responded to the NSC letter last week by saying the book contained nothing classified top secret.
“We do not believe that any of that information could reasonably be considered classified,” Mr Cooper wrote in an email to the White House on 24 January, the Washington Post reported.
Mr Cooper also said he had asked for an expedited review of a chapter about Ukraine, adding that Mr Bolton was “preparing” for the possibility he could be called to testify in the trial.