Trump's trial on charges approved on December 18 by the Democratic-led House of Representatives accusing him of abuse of power and obstruction of Congress was due to begin in the Senate on Tuesday.

On Monday, Senate Republican Majority Leader Mitch McConnell proposed rules that would execute a potentially quick trial.

He unveiled a resolution that would give House Democratic prosecutors and Trump lawyers 48 hours, evenly split, to present their arguments over a maximum of four days.

But the lead House impeachment manager in Trump's Senate trial, Adam Schiff, speaking just hours before the trial was due to begin, said the Republicans' proposed resolution governing the trial "does not prescribe a process for a fair trial."

Schiff said that if McConnell "makes this the first impeachment trial in history without witnesses or documents, it will not prove the president innocent - it will merely prove the Senate guilty of working with the president to obstruct the truth from coming out.

"This is not a process for a fair trial. This is the process for a rigged trial. This is the process if you do not want the American people to see the evidence."

Under the resolution, lawyers for Trump could move early in the proceedings to ask senators to dismiss all charges, a senior Republican leadership aide said, a motion that would likely fall short of the support needed to succeed.

Senate Democratic Minority Leader Chuck Schumer told CNN that Democrats would seek amendments.

House Speaker Nancy Pelosi, in a statement, said the Republicans' plan would force the trial into the "dark of night" and accused McConnell of choosing "a cover-up for the president, rather than honour his oath to the Constitution."

The trial is expected to continue six days a week, Monday through Saturday, until at least the end of January.

Australian Associated Press