By Patricia Zengerle and Susan Cornwell

WASHINGTON (Reuters) – The U.S. Senate is expected to wrap up the initial phase of President Donald Trump’s impeachment trial on Thursday before turning to the explosive question of whether to call witnesses such as former national security adviser John Bolton.

Republicans, who control the Senate, said there was a chance the trial could end on Friday with Trump’s acquittal on abuse of power and obstruction of Congress charges approved by the Democratic-led House of Representatives in December.

An acquittal would leave the Republican president in office and allow him to claim vindication just as the Democratic Party holds its first nominating contest for the Nov. 3 election in Iowa on Monday. Trump will hold a rally in the state on Thursday night.

Democrats accuse Trump of abusing his power by using congressionally approved military aid as leverage to get a foreign power to smear a leading contender for the Democratic nomination, former Vice President Joe Biden.

Lawyers for Trump and the House Democrats who are managing the impeachment prosecution will spend a second day on Thursday answering questions about the case written down by lawmakers and read aloud by U.S. Chief Justice John Roberts.

Then, probably on Friday, each side will present what amount to closing arguments before the senators move to the central question of whether to call witnesses to shed more light on Trump’s attempt to persuade Ukraine President Volodmyr Zelenskiy to investigate Biden.

Democrats need to persuade at least four Republican senators to vote with them to assure a majority vote in the 100-seat chamber, an effort the top Democrat in the Senate has called an uphill fight.

At least four Republicans – Lisa Murkowski of Alaska, Mitt Romney of Utah, Susan Collins of Maine and Lamar Alexander of Tennessee – and two Democrats – West Virginia’s Joe Manchin and Alabama’s Doug Jones – are seen as potentially on the fence on the issue.

Democrats are unlikely to muster the two-thirds majority needed to remove Trump from office no matter what happens, but allowing witnesses could inflict political damage on the president as he seeks re-election.

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 Biden.

Representative Sylvia Garcia, one of the Democratic impeachment managers, told reporters on Thursday morning she still thought it was possible that Bolton would appear.

“Today our job will be to convince them (the senators) that this will be a fair trial,” Garcia said on a conference call.

PRESIDENTIAL POWER

In questioning on Wednesday, Trump lawyer Alan Dershowitz offered an expansive defense of presidential power, saying: “If a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in an impeachment.”

Representative Adam Schiff, the lead House prosecutor, responded on Thursday in a tweet: “We’ve seen a remarkable lowering of the bar. According to Trump’s lawyers, everything is okay as long as the president believes it helps his reelection. It’s not okay to solicit foreign election interference, even if you fail. It just makes you a failed crook.”

Dershowitz tweeted on Thursday that the media had “distorted” his remarks.

Senator John Barrasso, the No. 3 Republican in seniority, has said it was possible the trial could end on Friday without witnesses being called in spite of pressure from Democrats.

“The momentum is clearly in the direction of moving to final judgment on Friday,” he said. Other Republican senators were predicting a similar outcome.

Democrats were not conceding defeat, however.

“There’s tremendous pressure from a vindictive, nasty president on every Republican senator, but I think (as) they sit there … we’ve got a real shot to get witnesses and documents,” Senate Democratic leader Chuck Schumer said on Wednesday.

If the vote on whether to allow witnesses is 50-50, Chief Justice Roberts could step in to break the tie. There is so little precedent for impeachment trials – this is only the third of a president in U.S. history – Senate aides said there is no way to know exactly what would occur, however.

(Additional reporting by Richard Cowan, David Morgan and Mohammad Zargham; Writing by Sonya Hepinstall; Editing by Tim Ahmann and Paul Simao)