WASHINGTON (BLOOMBERG) - President Donald Trump's impeachment trial formally opened in the Senate with Majority Leader Mitch McConnell prepared to push through ground rules that could wrap up the proceedings within days as Democrats vowed to fight him at every turn.

McConnell said a majority in the GOP-controlled chamber already backs his resolution setting out the terms of the trial that "tracks closely with past precedents".

He made last-minute changes to the final document offered in the Senate on Tuesday to extend arguments by the House and Trump's defence team over three days each rather the two days he initially proposed.

"The Senate's fair process will draw a sharp contrast with the unfair and precedent-breaking inquiry that was carried on by the House of Representatives," McConnell said on the Senate floor.

Senate Democratic leader Chuck Schumer said he will propose amendments to the trial rules, primarily to introduce witnesses and new evidence.

"The McConnell rules seem to be designed by President Trump, for President Trump," Schumer said at a news conference. "It's a cover up."

The trial, presided over by US Chief Justice John Roberts, opened with a reading of the resolution and initial statements regarding the rules by Democratic Representative Adam Schiff for the House impeachment managers and White House Counsel Pat Cipollone for the president's defence team.

"We believe that once you hear those initial presentations the only conclusion will be that the president has done absolutely nothing wrong," Cipollone said.

"We respectively ask you to adopt this resolution so we can begin this process."

McConnell's resolution gives House impeachment managers and Trump's defence team 24 hours each to make their cases over three days per side.

In another change from his initial plan, House impeachment inquiry evidence will be accepted by the Senate.

It defers the question of calling witnesses until after opening arguments and 16 hours of questioning by senators are completed.

SHOW OF UNITY

Wielding a 53-47 Senate majority, Republicans will have a quick chance for a powerful early show of partisan unity behind a GOP leader who publicly vowed to coordinate strategy with the White House.

Trump's eventual acquittal appears virtually assured, with a super-majority of 67 votes needed to convict him and not a single GOP senator suggesting Trump deserves to be removed from office.

The trial's real wild card is its political impact in a year when control of the White House and both chambers of Congress are in play.

The presidential impeachment trial, only the third in US history, comes amid polling that shows most Americans back bringing in new witnesses and other evidence.

More than three in four Americans say Trump administration officials, and the president himself, should be invited to testify before the Senate, according to a poll from Monmouth University released on Tuesday.

Two other recent polls showed similar results. The Monmouth survey also found that 57 per cent of Americans say the House impeachment managers should be able to introduce new evidence to support the two articles of impeachment.

The White House and impeachment managers from the House of Representatives released filings on Monday in which both sides argued that constitutional separation of powers is at stake in the trial.

The president's 171-page filing contends that the House failed to prove that the president explicitly linked aid for Ukraine to an investigation Trump sought into political rival and former vice-president Joe Biden, who is seeking to challenge Trump in the November election. The president's lawyers argued that the Senate should swiftly reject the impeachment articles.

The White House also announced that eight House Republicans who have been among the president's most vocal defenders would serve as part of his team "working to combat this hyper-partisan and baseless impeachment."

They include Representatives Doug Collins, Jim Jordan, Mark Meadows and Elise Stefanik, all of whom were prominent during the House's impeachment investigation and have made frequent media appearances on behalf of the president.

They will provide guidance for the defence and push Trump's case in the media, according to an administration official.

TRUMP'S LAWYER

House impeachment managers had a few strategic moves of their own. In a letter on Tuesday, they said Cipollone, who is Trump's lead defence lawyer, is himself a "fact witness" who must turn over evidence and whose trial involvement raises ethical questions.

"You must disclose all facts and information as to which you have first-hand knowledge that will be at issue in connection with evidence you present or arguments you make in your role as the president's legal advocate so that the Senate and chief justice can be apprised of any potential ethical issues, conflicts, or biases," they wrote to Cipollone.

White House spokesman Hogan Gidley dismissed the move as "completely absurd."

McConnell said previously that the rules for the process would closely track the rules of Bill Clinton's 1999 impeachment trial. At the last minute, he changed the number of days each side could use to present their cases to three days rather than two, matching the format of the Clinton trial.

He also changed the resolution from its initial draft to allow the House impeachment inquiry material into evidence unless there's an objection, also more closely tracking the precedent from Clinton.

McConnell's late changes came immediately after a closed door meeting with all Senate Republicans to discuss his proposal. Senators first saw the initial resolution when it was circulated Monday evening when both chambers were out of session for the Martin Luther King holiday.

McConnell's proposal doesn't specifically include a motion to dismiss the charges against Trump, after the majority leader said there wasn't enough backing in the Senate to pass it. However, that option remains and the White House has said it is reserving its right to utilise it.

Schumer said one aspect of McConnell's resolution that Democrats find particularly objectionable is a proposal to have a vote late in the trial about whether or not to subpoena documents and more witnesses.

Only if that succeeds, he pointed out, would the Senate go on to debate who should come.

The added "obstacle" he said, has the potential affect of voiding the big fight on witnesses Democrats want.

McConnell initially pressed his rank and file to coalesce behind a plan to have no witnesses, but he bowed to the demands of four Republican senators who insisted that there at least be a vote on whether to call witnesses.

Those senators are Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah and Lamar Alexander of Tennessee.

Collins, Murkowski and Romney have each indicated they could be inclined to vote with Democrats on specific witnesses, but they haven't committed to doing so.

Romney was among the GOP senators who indicated he will vote with McConnell on the initial rules. He said he would oppose any attempt to vote on witnesses early on, adding that he "made clear to my colleagues and the public that the Senate should have the opportunity to decide on witnesses following the opening arguments, as occurred in the Clinton trial."