Donald Trump’s impeachment trial in the Senate was dominated on its opening day by arguments about how the hearing should be conducted.
In an early sign of partisanship, the Republican-controlled Senate rejected three Democrat bids to force the White House to produce extra documents and evidence.
The president, who is 4,000 miles away from Washington in Davos, Switzerland, is charged with abuse of power and obstruction of Congress after investigations by the lower House of Representatives, which the Democrats control.
The case has now moved to the upper house which will decide whether he is guilty. But the final vote is expected to be along party lines and it is therefore unlikely he will be removed from office.
Mr Trump is accused of trying to pressure Ukraine into investigating his political rival, Democrat Joe Biden, by threatening to withhold military aid.
The US president has dismissed the charges as a “hoax” and a “witch-hunt”.
His top lawyer in the trial, Pat Cipollone, attacked the case as baseless, while leading Democratic politician Adam Schiff said there was “overwhelming” evidence of wrongdoing.
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In another sign of political wrangling, Senate majority leader Mitch McConnell, a Republican, had proposed each side would have 24 hours to present their opening arguments, spread over two 12-hour days.
Democrats opposed that, saying the rules would mean some arguments would be pushed into the early hours – when fewer people are watching – and make it harder to introduce evidence.
A few minutes in, Republicans appeared to relent. Legal teams for both sides will be able to spread their arguments over three 8-hour days.
Democrats also want to hear from witnesses during the trial, which the White House team is against.
The McConnell proposals put off any votes on whether witnesses can give evidence until later in the process, rather than at the start, as Democrats had demanded.
Mr Schiff told reporters before the trial got under way: “This is not a process for a fair trial, this is the process for a rigged trial” and called it a “cover-up”.
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Then at the start of the hearing, he said: “Right now a great many, perhaps even most Americans, do not believe there will be a fair trial.
“They don’t believe that the Senate will be impartial. They believe the result is pre-cooked, the president will be acquitted. Not because he is innocent, he is not, but because the senators will vote by party and he has the votes.
“The votes to prevent the evidence from coming out, the votes to make sure the public never sees it. The American people want a fair trial.”
Mr Schiff said that although the evidence against Mr Trump was “already overwhelming,” further witness testimony was necessary to show the full scope of the misconduct by the president and those around him.
But White House counsel Pat Cipollone said: “We believe that once you hear those initial presentations the only conclusion will be that the president has done absolutely nothing wrong.
“You look at those articles alone and you will determine that there is absolutely no case.”
Democrats claim Mr Trump abused his power in pressuring Ukraine, and obstructed Congress when it tried to find out what happened.
After opening arguments, senators will be allowed up to 16 hours for questions to the prosecution and defence, followed by four hours of debate.
Senate rules say the trial must proceed six days a week – excluding Sundays – until it is resolved.
Only two other US presidents in history have been impeached – Andrew Johnson in 1868 and Bill Clinton in 1998.
Both Mr Johnson and Mr Clinton were acquitted in the Senate, while in 1974 Richard Nixon faced impeachment but resigned before a full vote in the House of Representatives.