The US Senate has adopted ground rules for President Donald Trump’s impeachment trial after nearly 13 hours of rancorous debate on the first day.
Democrats sparred with White House lawyers over what would constitute a fair trial, while Republicans blocked their attempts to call new evidence.
The trial is set to resume on Wednesday with arguments by Democrats, to be followed by the defence and questions.
Mr Trump is the third US president to undergo an impeachment trial.
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He is charged with abuse of power and obstructing the congressional inquiry. He denies wrongdoing.
Mr Trump is being put on trial after being 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 and remove him from office.
At the World Economic Forum in Davos, Switzerland, on Tuesday, Mr Trump dismissed the accusations against him as “just a hoax”.
Republicans and Democrats argued late into the night after the trial got under way on Tuesday, with proceedings finishing close to 02:00 local time (07:00 GMT) on Wednesday.
Senators have taken oaths to act as impartial jurors in a trial presided over by the US Chief Justice, John Roberts.
Under the trial rules adopted, opening arguments will begin later on Wednesday, with each side being given up to 24 hours to lay out their case, over three days.
But the key issue of witnesses has still not been settled, with the debate and vote over whether they are called postponed until after each side has argued its case.
Democrats want to hear from key White House aides who worked closely with Mr Trump, including acting chief of staff Mick Mulvaney and the former national security adviser, John Bolton.
By party-line votes of 53-47, the Senate rejected three Democratic bids on Tuesday 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”.
First, the president is accused of seeking help from Ukraine’s government to help himself get re-elected in November.
It is claimed that, during a call with Ukrainian President Volodymyr Zelensky, he held back military aid as he sought an anti-corruption investigation into Democratic White House candidate Joe Biden, whose son, Hunter, held a board position with a Ukrainian energy firm, Burisma.
The second allegation is that, by refusing to allow White House staff to testify at the impeachment hearings last year, Mr Trump obstructed Congress.
The Senate is hearing the case as the Democratic-led House voted to impeach Mr Trump on 18 December.