U.S. Democrats signalled Monday they were girding for battle over witnesses and fair process in the Senate trial of Donald Trump, days ahead of a historic House vote on impeaching the president for Abuse of Office and Obstruction of Congress.

Top Senate Democrat Chuck Schumer said he was aiming for proceedings to start January 6 that would mete out "swift but fair justice" to Trump, even as Republican loyalists acknowledged they were less interested in being impartial jurors than protecting the president.

Lawmakers were beginning a consequential week. House Judiciary Committee Chairman Jerry Nadler released a 658-page report Monday outlining the case for impeaching Trump and detailing his alleged wrongdoing, including pressuring Ukraine to investigate Democrats.

It alleged severe episodes of "criminal" conduct by the president including bribery -- rebutting the Republican argument that Democrats have identified no specific criminal wrongdoing by Trump.

"President Trump's abuse of power encompassed the constitutional offence of 'bribery' and multiple federal crimes," it said, adding, Trump's conduct was "unlike anything this nation has ever seen."

The House Rules Committee was set to meet Tuesday to lay down guidelines for a floor debate on impeachment.

When the Democratic-controlled House convenes Wednesday to weigh the two charges approved by the Judiciary Committee, Trump is expected to become only the third US president to be impeached, after Andrew Johnson in 1868 and Bill Clinton in 1998.

Richard Nixon resigned in 1974 just before a House impeachment vote. Neither Johnson nor Clinton was convicted in the Senate.

'Just the facts'

Trump is also unlikely to be removed from office by the Senate, where Republicans hold a 53-47 majority.

But Schumer has pressed hard for a fair process, writing Senate Majority Leader Mitch McConnell to demand four key witnesses testify, including acting White House chief of staff Mick Mulvaney and Trump's ex-national security advisor John Bolton.

Schumer also sought to set limits on testimony length and questioning of witnesses, proposing a structure that would give Americans what he called "confidence in the process."

"Just the facts. We don't need fishing expeditions," Schumer told CNN. "We're trying to have the kind of justice America is known for, which is swift but fair justice."

Senate rules on impeachment are determined by a simple majority vote in the chamber. Although Schumer is looking to strike a deal with McConnell on the rules, it will be the will of the majority that wins out.

Democrats have bridled at McConnell's recent promise of "total coordination" with the White House, as well as Senator Lindsey Graham's apparent dismissal of the need to be an impartial juror in the process.

"I'm not trying to pretend to be a fair juror here," the Trump loyalist said Sunday, rejecting the charges against Trump as "partisan nonsense."

One of the two impeachment articles to go before the House charges Trump with abuse of power for conditioning military aid and a White House meeting on Ukraine's announcing investigations into Democrats ahead of the 2020 presidential election.

The other charges him with obstructing Congress for refusing to cooperate with the inquiry and ordering other officials not to appear, a development Democrats say is unprecedented in American history.

The president has repeatedly assailed the process and the Democrats conducting it.

"The Impeachment Hoax is the greatest con job in the history of American politics!" he boomed Monday on Twitter.

The impeachment hearings have been a sometimes grim exercise for Democrats, who fear moderate members of the party from Trump-friendly districts could lose their seats next year if they vote to impeach.

One Democrat opposed to impeachment, New Jersey's Jeff Van Drew, is expected to switch allegiance to the Republican Party this week.

A handful of freshman Democrats are mulling opposing impeachment. One of them, former CIA officer Elissa Slotkin, said Monday she would vote to impeach Trump.

If a president admits to inviting foreign interference in US elections and "solicits additional help from even more capable foreign governments (including China) then isn't it our constitutional duty to provide a clear response to that abuse of power?" she wrote in the Detroit Free Press. (AFP)

•Embattled U.S. President Trump

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