Washington, Jan 21 (AP) President Donald Trump's impeachment trial quickly burst into a partisan fight on Tuesday at the Capitol as Democrats objected strongly to rules proposed by the Republican leader for compressed arguments and a speedy trial.

Chief Justice John Roberts g aveled open the session, senators having taken an oath last week to do "impartial justice" as jurors.

Democrats warned that the rules package from Trump's ally, Senate Majority Leader Mitch McConnell, could force midnight sessions that would keep most Americans in the dark and create a sham proceeding.

"This is not a process for a fair trial, this is the process for a rigged trial" Rep. Adam Schiff, D-Ca., the chairman of the House Intelligence Committee leading the prosecution, told reporters.

He called it a "cover-up". McConnell opened the chamber promising a "fair, even handed" process — and warned that the Senate would stay in session until his proposed rules package was adopted.

"The president's lawyers will finally receive a level playing field," the Kentucky Republican said, contrasting it with the House impeachment inquiry.

The first test was coming as senators prepared to begin debate and vote on McConnell's proposed rules. The package diverges in some ways from the most recent impeachment trial, of Bill Clinton, by condensing opening arguments into two days for each side. It pushes votes to consider whether or not to hear from witnesses later in the process.

Republican senators, who hold the majority, are falling in line behind his plan.

"Sure it will be a fair trial when you've got 24 hours of arguments on both sides," Senator Chuck Grassley of Iowa told state reporters on a conference call.

The rare impeachment trial, unfolding in an election year, is testing whether Trump's actions toward Ukraine warrant removal at the same time that voters are forming their own verdict his White House.

The Democrats say the prospect of middle-of-the-night proceedings, without allowing new witnesses or even the voluminous House records of the trial, will leave the public without crucial information about Trump's political pressure campaign on Ukraine and the White House's obstruction of the House impeachment probe.

"The McConnell rules seem to be designed by President Trump for President Trump," said the Senate Democratic leader, Chuck Schumer.

He vowed to call for a series of votes to amend the rules and demand testimony and documents, but it seemed unlikely Republicans would break from the party to join Democrats.

"This is a historic moment," Schumer said. "The eyes of America are watching. Republican senators must rise to the occasion."

If the senators agree to McConnell's proposal for speedy trial and acquittal, Schiff said, "It will not prove the president innocent, it will only prove the Senate guilty of working with the president to obstruct the truth from coming out."

Rep. Jerry Nadler, the Judiciary Committee Chairman also leading the House team, said: "There's no trial in this country where you wouldn't admit relative witnesses."

McConnell had promised to set rules similar to the last impeachment trial, of President Bill Clinton in 1999, but his resolution diverged in key ways.

GOP Senator Mitt Romney of Utah, whose votes are being closely watched, said he was satisfied with the proposal, even as he hopes to hear from former national security adviser John Bolton who had a front-row seat to Trump's actions.

The first several days of the trial are expected to be tangled in procedural motions playing out on the Senate floor and behind closed doors. Senators must refrain from speaking during the trial proceedings.

Like Trump, House Speaker Nancy Pelosi will also be away for the proceedings, leading a bipartisan congressional delegation to Poland and Israel to commemorate the 75 years since the liberation of Auschwitz at the end of World War II.

She issued a statement on Tuesday denouncing McConnell's proposed ground rules as a "sham" because of the compressed schedule and lack of guarantee that witnesses will be called or that evidence gathered by the House would be admitted in the Senate trial.

"The Senate GOP Leader has chosen a cover-up for the President, rather than honour his oath to the Constitution," Pelosi said.

House Democrats impeached the Republican president last month on two charges: abuse of power by withholding US military aid to Ukraine as he pressed the country to investigate Democratic rival Joe Biden, and obstruction of Congress by refusing to cooperate with their investigation.

The Constitution gives the House the sole power to impeach a president and the Senate the final verdict by convening as the impeachment court for a trial.

Four TV monitors were set up inside the Senate chamber to show testimony, exhibits and potentially tweets or other social media, according to a person familiar with the matter but unauthorised to discuss it on the record.

No president has ever been removed from office by the Senate. With its 53-47 Republican majority, the Senate is not expected to mount the two-thirds voted needed for conviction.

Even if it did, the White House team argues it would be an "unconstitutional conviction" because the articles of impeachment were too broad. (AP)

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