The US Senate on Friday rebuffed Democratic calls for new witnesses at President Donald Trump's impeachment trial, paving the way for his acquittal next week of charges of abuse of power and obstruction of Congress.

Following the 51-49 vote, the Senate scheduled four hours of closing arguments by Democratic prosecutors and White House defense lawyers on Monday and a vote for acquittal on Wednesday.

US President Donald Trump
In Michigan, US President Donald Trump said the impeachment trial of him is a "disgrace". Photo: AFP / SAUL LOEB

A Wednesday vote will mean the historic trial will not have concluded before Tuesday, when Trump is scheduled to give his nationally televised annual "State of the Union" speech to a joint session of Congress.

Trump is all-but-assured of being acquitted by the Senate, where Republicans hold 53 seats to 47 for the Democrats and a two-thirds majority -- or 67 senators -- is needed to remove a president from office.

Chief Democratic prosecutor Adam Schiff
Chief Democratic prosecutor Adam Schiff answers questions from senators at the impeachment trial of President Donald Trump. Photo: US Senate TV / HO

Two Republican senators -- Mitt Romney of Utah and Susan Collins of Maine -- joined 47 Democrats in voting to introduce further witnesses into the trial.

But Democrats failed to muster the four votes needed to allow testimony from Trump's former national security advisor John Bolton, acting White House chief of staff Mick Mulvaney and others.

Senate majority leader Mitch McConnell
Senate Majority Leader Mitch McConnell could call for a vote to acquit or convict President Donald Trump as early as January 31, 2020. Photo: US Senate TV / HO

Senate Democratic Minority Leader Chuck Schumer called it a "grand tragedy."

"America will remember this day --- a day when the United States Senate did not live up to its responsibilities, turned away from truth and instead went along with a sham trial," Schumer said.

Chief Democratic prosecutor Adam Schiff
Chief Democratic prosecutor Adam Schiff answers questions at the Senate impeachment trial of President Donald Trump. Photo: US Senate TV / HO

Nancy Pelosi, the Democratic speaker of the House of Representatives, which impeached Trump on December 18, accused Republicans who voted against allowing witnesses of being "accomplices to the president's cover-up."

US Supreme Court Chief Justice John Roberts
US Supreme Court Chief Justice John Roberts is presiding over the Senate impeachment trial of President Donald Trump. Photo: US Senate TV / HO

"He is impeached forever," Pelosi said. "There can be no acquittal without a trial. And there is no trial without witnesses, documents and evidence."

Democrats had been eager to hear from Bolton following reports that he claims in an upcoming book to have been personally told by Trump that military aid to Ukraine was tied to Kiev investigating Joe Biden, his potential Democratic rival in November's presidential election.

Trump's impeachment trial
US President Donald Trump's impeachment trial resumes in the Senate, with senators posing questions to the House managers and the president's defense lawyers about Trump's conduct toward Ukraine and the articles of impeachment against him. Photo: Senate Pool /

The charge is the crux of the case which led to Trump becoming just the third president in US history to be impeached by the House of Representatives.

Republican Senate Majority Leader Mitch McConnell, a Trump ally, said House prosecutors, known as "managers," had already presented enough evidence to make their case and there was no need for further witnesses.

"A majority of the US Senate has determined that the numerous witnesses and 28,000-plus pages of documents already in evidence are sufficient to judge the House Managers' accusations and end this impeachment trial," he said.

Republican Senator Lamar Alexander
Republican Senator Lamar Alexander said he would oppose calling new witnesses at the Senate impeachment trial of President Donald Trump. Photo: GETTY IMAGES / Samuel Corum

"There is no need for the Senate to re-open the investigation," he said. "Never in Senate history has this body paused an impeachment trial to pursue additional witnesses."

McConnell said the Senate would resume as a court of impeachment at 11:00 am (1600 GMT) on Monday to allow House prosecutors and White House lawyers to deliver their final arguments. Each side will have two hours to speak.

The Senate will then vote at 4:00 pm on Wednesday on the two articles of impeachment passed last month by Democrats in the House.

With the focus shifting to the final vote, all eyes will be on whether any Democrats facing potential tough re-election battles in November will vote to acquit the president.

Democratic Senator Doug Jones, for example, is facing what is shaping up to be a tough re-election fight in Alabama, a state which voted overwhelmingly for Trump in the 2016 election.

Democrats had been hoping to sway enough Republicans to support the call for further witnesses but efforts collapsed when Alaska Senator Lisa Murkowski said she would vote "no" to new testimony.

"Given the partisan nature of this impeachment from the very beginning and throughout, I have come to the conclusion that there will be no fair trial in the Senate," Murkowski said.

"I don't think the continuation of this process will change anything," she added.

Republican Senator Lamar Alexander of Tennessee, who had also been considered a possible swing vote, also said there was no need for more evidence.

"It was inappropriate for the president to ask a foreign leader to investigate his political opponent and to withhold United States aid to encourage that investigation," Alexander said.

"But the Constitution does not give the Senate the power to remove the president from office and ban him from this year's ballot simply for actions that are inappropriate," he said.

Before the Senate vote, Adam Schiff, the leader of the House impeachment prosecutors, made an impassioned last-ditch argument for witnesses.

"A trial without witnesses is simply not a trial," the California lawmaker said.

"This trial is supposed to be a quest for the truth," he said. "Let's not fear what we will learn."

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