Trump acquittal 'meaningless' without witnesses: top Senate Democrat

US President Donald Trump's expected acquittal in his impeachment trial will be a hollow victory if the trial does not call any witnesses, top Senate Democrat Chuck Schumer said Friday.

"This country is headed towards the greatest cover-up since Watergate," Schumer said ahead of what could be the final day of the historic hearings.

"If my Republican colleagues refuse to consider witnesses and documents in this trial, the president's acquittal will be meaningless. Because it will be the result of a sham trial."

Schumer appeared resigned to Trump being acquitted by the Republican-controlled Senate on charges of abuse of power and obstruction of Congress, after Democrats looked set to fail to convince enough Republicans to side with them on calling witnesses.

With only 47 of the 100 seats in the Senate, Democrats need four Republicans to cross over to be able to call witnesses and prolong the trial.

Ahead of the final four hours of scheduled debate in the trial, there were signs that they might have three at best.

"Today the Senate will vote on whether witnesses and documents are allowed in this trial. The importance of this vote is self-evident. Trials have witnesses, trials have documents," Schumer said.

If the vote on witnesses fails, Republican Majority Leader Mitch McConnell, a close trump ally, is expected to try to move quickly to a final vote on the charges.

That could be as early as Friday night, though it could be delayed by further debate.

Trump's acquittal is all but guaranteed, given the two-thirds super-majority required to convict him.