White House lawyers have argued for a swift rejection of “flimsy” charges against Donald Trump on the eve of the historic impeachment trial against the US president.

As the Capitol awaited the start of the contentious proceedings – which are unfolding in an election year – the Senate leader also proposed a compressed calendar for opening statements.

Final trial preparations were underway on Monday on a tense day of developments, with Mr Trump’s legacy — and the judgment of both parties in Congress — at stake.

The president’s legal team, in its first full filing for the impeachment court, argued Mr Trump did “absolutely nothing wrong” and urged the Senate to swiftly reject the “flawed” case against him.

“All of this is a dangerous perversion of the Constitution that the Senate should swiftly and roundly condemn,” the president’s lawyers wrote. “The articles should be rejected and the president should immediately be acquitted.”

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Donald Trump glances at journalists as he leaves the White House on Monday on the eve of his impeachment trial (Manuel Balce Ceneta/AP)

The brief from the White House, and the House Democratic response, came as it emerged that 12-hour sessions were on the agenda for the rare Senate trial, with some of the very senators running to replace Mr Trump as president sitting as jurors.

Senate Majority Leader Mitch McConnell proposed a condensed, two-day calendar for each side to give opening statements, ground rules that Democrats immediately rejected.

Voting on the Republican leader’s resolution will be one of the first orders of business when senators convene on Tuesday. It also pushes off any votes on witnesses until later in the process, rather than up front, as Democrats had demanded.

The Senate Democratic leader, Chuck Schumer, called the GOP leader’s proposed rules package a “national disgrace”.

Senators are poised for only the third presidential impeachment trial in US history, coming just weeks before the first primaries of 2020 election season and as voters assess Mr Trump’s first term and consider the candidates who want to challenge him in the autumn.

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

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The Democrats’ Senate Minority Leader Chuck Schumer, seen here last week, has accused Republicans of trying to rush the impeachment proceedings (Julio Cortez/AP)

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.

Mr McConnell is angling for a speedy trial towards acquittal, and with Republicans holding the Senate majority, the proposal is likely to be approved by senators in the president’s party.

“It’s clear Senator McConnell is hell-bent on making it much more difficult to get witnesses and documents and intent on rushing the trial through,” said Mr Schumer, who vowed to propose votes on Tuesday to try to amend the rules package.

The first several days of the trial are now almost certain to be tangled in procedural motions playing out on the Senate floor or, more likely, behind closed doors, since senators must refrain from speaking during the trial proceedings.

At the White House, where the president was embarking for an overseas trip to the global leaders conference in Davos, Switzerland, officials welcomed the Republican trial proposal.

“We are gratified that the draft resolution protects the president’s rights to a fair trial, and look forward to presenting a vigorous defence on the facts and the process as quickly as possible, and seeking an acquittal as swiftly as possible,” said White House Legislative Affairs Director Eric Ueland.

After the four days of opening arguments — two days per side — senators will be allowed up to 16 hours for questions to the prosecution and defence, followed by four hours of debate. Only then will there be votes on calling other witnesses.

At the end of deliberations, the Senate would then vote on each impeachment article.

Mr McConnell had promised to set rules similar to the last trial, of President Bill Clinton in 1999, but his resolution diverged in key ways, which may leave some senators from both parties uneasy.

Republican Senator Mitt Romney of Utah said in an email message to his constituents on Monday that the resolution put forth by Mr McConnell “overall, aligns closely with the rules package approved 100-0 during the Clinton trial”.

Mr Romney is among a small number of Republican senators who want to consider witness testimony and documents that weren’t part of the House impeachment investigation.

In their filing on Monday, House prosecutors issued fresh demands for a fair trial in the Senate.

“President Trump asserts that his impeachment is a partisan ‘hoax.’ He is wrong,” the prosecutors wrote.

The White House document released Monday says the two charges against the president don’t amount to impeachable offences. It asserts the impeachment inquiry, centred on Mr Trump’s request that Ukraine’s president open an investigation into Democratic rival Mr Biden, was never about finding the truth.

House Democrats in their initial court filing over the weekend called Mr Trump’s conduct the “worst nightmare” of the founding fathers who framed the Constitution.

But Mr Trump’s team contended Monday that even if Trump were to have abused his power in withholding the Ukraine military assistance, it would not be impeachable because it did not violate a specific criminal statute.