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 defence lawyers on Monday and a vote for acquittal on Wednesday.READ | Senate poised to reject witnesses, acquit Trump of impeachment chargesA 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.Two Republican senators - Mitt Romney of Utah and Susan Collins of Maine - joined 47 Democrats in voting to introduce further witnesses into the trial.'Grand tragedy'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 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.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"."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.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.