The Republican-controlled US Senate has blocked Democratic attempts to obtain evidence and call a witness in President Donald Trump's impeachment trial, an early sign the proceeding could advance on lines favourable to Trump.
As the third presidential impeachment trial in US history began in earnest on Tuesday, Trump's chief legal defender described the case as a baseless effort to overturn the 2016 election and a top Democratic lawmaker said there was "overwhelming" evidence of wrongdoing.
After US Chief Justice John Roberts convened the proceedings, the two sides quickly began squabbling over Senate Majority Leader Mitch McConnell's proposed rules for a trial to determine if Trump should be removed from office.
Trump, who was impeached last month by the Democratic-led House of Representatives on charges of abuse of power and obstruction of Congress, is accused of pressuring Ukraine to investigate former Democratic Vice President Joe Biden, a political rival, and then impeding the inquiry into the matter.
The president denies any wrongdoing and describes his impeachment as a partisan hoax to derail his 2020 re-election effort.
Senators voted along party lines, 53-47, to block four separate motions from Democratic leader Chuck Schumer to subpoena records and documents from the White House, the State Department, the Defence Department, and the Office of Management and Budget related to Trump's dealings with Ukraine.
Senators also rejected by the same tally a request for a subpoena seeking the testimony of acting White House Chief of Staff Mick Mulvaney.
White House counsel Pat Cipollone, who is leading Trump's defence, attacked the foundation of the charges against the Republican president and said Democrats had not come close to meeting the US Constitution's standard for impeachment.
Cipollone argued in favour of McConnell's proposal to wait until later in the trial to decide whether to allow further witnesses or documents.
Democratic Representative Adam Schiff, who helped spearhead the House impeachment inquiry, said the president had committed "constitutional misconduct justifying impeachment."
While the evidence against Trump was "already overwhelming," further witness testimony was necessary to show the full scope of the misconduct by the president and those around him, he said.
Democrats said they forced repeated votes on evidence and witnesses, which extended late into the night, to get Republicans on the record immediately.
"This may be our only chance, tonight, to make this a fair trial. And it's just increasingly clear that the White House has no answers for why these documents and these witnesses shouldn't be produced," US Senator Chris Murphy of Connecticut told reporters.
McConnell proposed giving Democratic prosecutors and Trump's lawyers 24 hours each over three days for opening arguments, easing off an earlier plan to keep them to two days and also allowing the House's record of the probe to be admitted as evidence..
Under McConnell's plan, 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.
Trump is almost certain to be acquitted by the 100-member chamber, where a two-thirds majority is needed to remove him from office.
But the impact of the trial on his re-election bid in November is far from clear.
Democrats accuse Trump of pressuring Ukraine, a vulnerable ally, to interfere in US elections at the expense of American national security and say he is a danger to American democracy.
Trump and his legal team say there was no pressure and that the Democrats' case is based on hearsay.
The Senate trial is expected to continue six days a week, Monday through Saturday, until at least the end of January.
Australian Associated Press