The US Senate has rejected repeated Democratic attempts to secure new evidence in President Donald Trump's impeachment as his trial began.
Senate Majority leader Mitch McConnell meanwhile backed off on a plan to fast-track the hearings after coming under pressure from fellow Republicans.
Democrats said this would have been no less than a cover-up.
Mr Trump is charged with abuse of power and obstructing the congressional inquiry. He denies wrongdoing.
At the World Economic Forum in Davos, Switzerland, on Tuesday, Mr Trump dismissed the accusations against him as "just a hoax".
Senators have taken oaths to act as impartial jurors, hearing arguments for six hours a day, six days a week in a trial presided over by the US Chief Justice, John Roberts.
It is only the third time in US history that a president is facing an impeachment trial and it is unclear how long it will last.
Mr Trump was impeached last month by the Democratic-led House of Representatives.
But the Senate, which is controlled by his fellow Republicans, is not expected to convict the president and remove him from office.
On Tuesday, by party-line votes of 53-47, the Senate rejected three Democratic bids to obtain documents and evidence in the impeachment trial.
Senators blocked a motion from Democratic leader Chuck Schumer to subpoena White House files related to Mr Trump's dealings with Ukraine.
They also rejected follow-up motions demanding a subpoena of records and documents from the state department and White House budget office.
In his opening statement, Adam Schiff, the House Democrat leading the impeachment case, said most Americans "do not believe there will be a fair trial".
"They don't believe the Senate will be impartial," he added. "They believe the result is pre-cooked."
The president's legal team had earlier demanded he be immediately acquitted, calling the trial "a dangerous perversion of the constitution".