The United States Senate narrowly rejected Democratic demands to summon witnesses for President Donald Trump's impeachment trial late on Friday, all but ensuring Trump's acquittal in just the third trial to threaten a president's removal in the US history. But senators pushed off final voting on his fate to next Wednesday.

The delay in timing showed the weight of a historic vote bearing down on senators, despite prodding by the president eager to have it all behind him in an election year and in the run-up to his State of the Union speech Tuesday night.

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Trump and Senate Majority Leader Mitch McConnell spoke by phone to lock in the schedule during a tense night at the Capitol as rushed negotiations proceeded on and off the Senate floor. The trial came to a standstill for about an hour. A person unauthorised to discuss the call was granted anonymity to describe it.

The president wanted to arrive for his speech at the Capitol with acquittal secured, but that will not happen. Instead, the trial will resume on Monday for final arguments, with time on Monday and Tuesday for senators to speak. The final voting is planned for 4pm (21:00 GMT) on Wednesday, the day after Trump's speech.

Trump's acquittal is all but certain in the Senate, where his Republican allies hold the majority and there is nowhere near the two-thirds needed for conviction and removal; nor will he face potentially damaging, open-Senate testimony from witnesses.

Despite the Democrats' singular focus on hearing new testimony, the Republican majority brushed past those demands and will make this the first impeachment trial without witnesses. Even new revelations on Friday from former national security adviser John Bolton did not sway GOP senators, who said they had heard enough.

That means the eventual outcome for Trump will be an acquittal "in name only", said Democratic Representative Val Demings, a House prosecutor, during the final debate.

Trump was impeached by the House last month on charges that he abused power and obstructed Congress as he tried to pressure Ukraine to investigate Democratic rival Joe Biden, using military aid as leverage as the ally fought Russia. He is charged with then blocking the congressional probe of his actions.

Senators rejected the Democrats' effort to allow new witnesses, 51-49, a near party-line vote. Republicans Susan Collins of Maine and Mitt Romney of Utah voted with the Democrats, but that was not enough.

Senate Democratic leader Chuck Schumer called that decision "a tragedy on a very large scale". Protesters' chants reverberated against the walls of the Capitol.

But Republicans said Trump's acquittal was justified and inevitable.

"The sooner the better for the country," said Senator Lindsey Graham, a Trump confidant. "Let's turn the page."

The next steps come in the heart of the presidential campaign season before a divided nation. Democratic caucus voting begins on Monday in Iowa, and Trump gives his State of the Union address the next night. Four Democratic candidates have been chafing in the Senate chamber rather than campaigning.

Bolton revelations

The Democrats had badly wanted testimony from Bolton, whose forthcoming book links Trump directly to the charges. But Bolton will not be summoned, and none of this appeared to affect the trial's expected outcome. Democrats forced a series of new procedural votes late on Friday to call Bolton and White House acting chief of staff Mick Mulvaney, among others, but all were rejected.

In an unpublished manuscript, Bolton has written that the president asked him during an Oval Office meeting in early May to bolster his effort to get Ukraine to investigate Democrats, according to a person who read the passage and told The Associated Press news agency. The person, who was not authorised to disclose contents of the book, spoke only on condition of anonymity.

Trump listens as his then-NSA John Bolton speaks [File: Leah Millis/Reuters]

In the meeting, Bolton said the president asked him to call new Ukrainian President Volodymyr Zelenskyy and persuade him to meet Trump's personal lawyer, Rudy Giuliani, who was planning to go to Ukraine to coax the Ukrainians to investigate the president's political rivals. Bolton writes that he never made the call to Zelenskyy after the meeting, which included acting chief of staff Mick Mulvaney and White House Counsel Pat Cipollone.

The revelation adds more detail to allegations of when and how Trump first sought to influence Ukraine to aid investigations of his rivals that are central to the abuse of power charge in the first article of impeachment.

The story was first reported on Friday by The New York Times.

Trump issued a quick denial.

"I never instructed John Bolton to set up a meeting for Rudy Giuliani, one of the greatest corruption fighters in America and by far the greatest mayor in the history of NYC, to meet with President Zelenskyy," Trump said. "That meeting never happened."

'Congress has failed'

Key Republican senators said even if Trump committed the offences as charged by the House, they are not impeachable and the partisan proceedings must end.

"I didn't need any more evidence because I thought it was proved that the president did what he was charged with doing," retiring Republican Senator Lamar Alexander, a late holdout, told reporters on Friday at the Capitol. "But that didn't rise to the level of an impeachable offence."

Republican Senator Lisa Murkowski said she, too, would oppose more testimony in the charged partisan atmosphere, having "come to the conclusion that there will be no fair trial in the Senate". She said, "The Congress has failed."

Eager for a conclusion, Trump's allies nevertheless suggested the shift in timing to extend the proceedings into next week, acknowledging the significance of the moment for senators who want to give final speeches.

Senate Majority Leader Mitch McConnell made the offer to Schumer.

Under the resolution, the Senate would resume on Monday for final arguments, with time on Monday and Tuesday for senators to speak. The final voting would be on Wednesday.

To bring the trial towards a conclusion, Trump's attorneys argued the House had already heard from 17 witnesses and presented its 28,578-page report to the Senate. They warned against prolonging it even further after House impeached Trump largely along party lines after less than three months of formal proceedings making it the quickest, most partisan presidential impeachment in the US history.

Some senators pointed to the importance of the moment.

"What do you want your place in history to be?" asked one of the House managers, Jason Crow, a former army ranger.

Trump is almost assured of eventual acquittal with the Senate nowhere near the 67 votes needed for conviction and removal.

The presiding officer, Chief Justice of the United States John Roberts listens [State TV/AP Photo]

To hear more witnesses, it would have taken four Republicans to break with the 53-seat majority and join with all Democrats in demanding more testimony. But that effort fell short.

Chief Justice John Roberts, in the rare role presiding over the impeachment trial, could have broken a tie, but asked late on Friday, he told senators it would be "inappropriate".

Murkowski noted in announcing her decision that she did not want to drag the chief justice into the partisan fray.

Though protesters stood outside the Capitol, few visitors have been watching from the Senate galleries.