The Republican-controlled US Senate will hear opening arguments in President Donald Trump's impeachment trial on Wednesday, beginning several days of argument on whether to remove Trump from office.
In Davos, Switzerland, Trump said allowing current and former top administration officials such as John Bolton to testify at the trial would present national security concerns.
In a 13-hour battle over trial rules that lasted until the wee hours of Wednesday, Republican senators rejected requests for subpoenas seeking the testimony of Bolton, Trump's former national security adviser, and three White House officials.
That battle is likely to resume next week.
Trump was impeached last month by the Democratic-run House of Representatives on charges of abuse of power and obstruction of Congress for pressuring Ukraine to investigate former Vice President Joe Biden, a Democratic political rival, and impeding a congressional inquiry into the matter.
The president denies any wrongdoing.
The trial, the third presidential impeachment trial in US history, was due to resume at 1 p.m. ET (1800 GMT). On Tuesday - effectively the trial's opening day - Democrats argued that more witnesses and records were needed since the Trump administration had not complied with requests for documents and urged officials not to participate.
Arguments became so heated that Chief Justice John Roberts, who is presiding over the trial, admonished both the defence and prosecution.
Representative Adam Schiff, leading the House Democrats' prosecution team of "managers," said the evidence against Trump was "already overwhelming" but further witness testimony was necessary to show the full scope of the misconduct by the president and those around him.
Trump is almost certain to be acquitted by the Republican-controlled 100-member chamber, where a two-thirds majority is needed to remove him from office. But the trial's effect on Trump's November re-election bid is unclear.
Trump said he would prefer a longer trial so that current and former top administration officials could testify, but that having them appear would present national security concerns.
"The problem with John (Bolton) is that it's a national security problem," Trump said at the World Economic Forum in Davos.
"He knows some of my thoughts, he knows what I think about leaders. What happens if he reveals what I think about a certain leader and it's not very positive?" he told a news conference.
Republican senators have not ruled out the possibility of further testimony and evidence at some point later in the trial but they held firm with Trump to block Democratic requests for witnesses and evidence.
"They insist that the president has done nothing wrong, but they refuse to allow the evidence and hearing from the witnesses ... and they lie, and lie and lie and lie," Representative Jerrold Nadler, another Democratic impeachment manager, said of Trump's lawyers in remarks to the Senate.
White House counsel Pat Cipollone fired back.
"Mr Nadler, you owe an apology to the president of the United States and his family," Cipollone said. "You owe an apology to the Senate. But most of all you owe an apology to the American people."
REMEMBER WHERE YOU ARE
That back-and-forth led Roberts, the chief justice of the United States, to admonish both men.
"I do think those addressing the Senate should remember where they are," he said.
Republicans could try to call Biden as a witness, an idea Democrats have resisted as a needless distraction.
During a debate that finally wrapped up near 2 a.m. (0700 GMT) on Wednesday, senators rejected by 53-47 votes motions from leading Senate Democrat Schumer to subpoena records and documents from the White House, the State Department, the Defense Department, and the Office of Management and Budget related to Trump's dealings with Ukraine.
Senators also rejected requests for subpoenas seeking the testimony of acting White House Chief of Staff Mick Mulvaney, White House aide Robert Blair and White House budget official Michael Duffey.
Under the rules, lawyers for Trump could move early in the proceedings to ask senators to dismiss all charges, according to a senior Republican leadership aide, a motion that would likely fall short of the support needed to succeed.
Schumer on Wednesday poured cold water on a report in the Washington Post that Democrats are privately considering a deal under which Biden and his son Hunter would testify in exchange for Bolton testifying.
Hunter Biden served on the board of a Ukrainian gas company and he is at the heart of unsubstantiated allegations by Trump of corruption in Ukraine.
The Senate trial is expected to run six days a week, Monday through Saturday, until at least the end of January.
Trump and his legal team say that there was no pressure on Ukraine and that the Democrats' case is based on hearsay. Cipollone described the Ukraine investigation as an illegal attempt to remove a democratically elected president and avert his re-election.
No president has ever been removed through impeachment, a mechanism the nation's founders - worried about a monarch on American soil - devised to oust a president for "treason, bribery or other high crimes and misdemeanours." One president, Richard Nixon, resigned in the face of a looming impeachment.