On the eve of Donald Trump's impeachment trial, his legal team Monday denounced the case as unconstitutional, calling it "absurd" to hold the former president responsible for the deadly riot at the US Capitol.

But in a preview of their prosecuting arguments, Democrats accused Trump of committing the "most grievous constitutional crime" in the 232-year history of the American presidency by inciting his supporters to storm Congress on January 6.

The House of Representatives impeached Trump for a historic second time last month over his role in the deadly siege, and his trial -- the first of a former president -- begins Tuesday with the Senate's 100 members sitting as jurors.

The chamber's leaders announced they and the legal teams had reached agreement on the rules of the process, which is expected to launch Tuesday at 1:00 pm (1800 GMT) with debate and a vote on the constitutionality of the trial itself.

Arguments will be heard from Wednesday, with each side provided 16 hours over two days.

Trump is a deeply damaged political figure but remains a powerful force in the Republican Party.

Charged with "incitement of insurrection," he is likely once again to avoid conviction due to loyal support in the Senate, but his lawyers contended in their final pre-trial filing that the Constitution does not give the chamber jurisdiction to try a former president.

"The Senate should dismiss these charges and acquit the president because this is clearly not what the framers wanted or what the Constitution allows," his attorneys Bruce Castor, David Schoen and Michael T. van der Veen wrote.

"Indulging House Democrats hunger for this political theater is a danger to our Republic democracy and the rights that we hold dear."

The defense used blunt language in their 78-page brief, saying it was "simply absurd" to argue that Trump conjured up a mob to commit violent crime, and that those who attacked the Capitol did so on their own.

The lawyers rejected as "patently ridiculous" the trial of a former president, a private citizen who cannot be removed "from an office that he no longer holds."

- 'Leave it to the Senate' -

President Joe Biden, who succeeded Trump on January 20, weighed in Monday but declined to address whether Trump should be found guilty or denied the right to hold political office in the future.

"We'll let the Senate work that out," Biden said.

White House spokeswoman Jen Psaki later told reporters that Biden ran against Trump in 2020 "because he felt he was unfit for office."

"But he's going to leave it to the Senate to see this impeachment proceeding through," Psaki added.

Should Trump be convicted, the Senate will then hold a simple-majority vote on barring him from future public office, Senate Majority Leader Chuck Schumer confirmed.

The congressional Democrats prosecuting the trial, known as House impeachment managers, provided their final counterarguments Monday, pushing back against Trump's legal contentions.

"The evidence of President Trump's conduct is overwhelming (and) his efforts to escape accountability are unavailing," they said in a five-page brief.

"His incitement of insurrection against the United States government -- which disrupted the peaceful transfer of power -- is the most grievous constitutional crime ever committed by a president."

 - Loyal conservatives - 

The proceedings will take place in the very Senate chamber that was raided by rioters, threatening the lives of lawmakers in an effort to stop the ceremonial certification of Biden's election victory.

The impeachment managers argued in an earlier brief that Trump, who rallied supporters in Washington shortly before the mob assault, was "singularly responsible" for the unrest which left five people dead.

And they said acquitting Trump -- who escaped conviction in his first impeachment trial in 2020 -- could do severe damage to American democracy.

But convicting him would require the vote of more than two-thirds of the senators, meaning 17 Republicans would need to break ranks and join all 50 Democrats -- seen as near impossible.

Even though Trump retains a strong base of support, the riot has eroded his popularity -- not good for a 74-year-old mulling a fresh presidential run in 2024.

Public support for a Trump conviction is stronger now than during his first impeachment trial, according to a new Ipsos/ABC News poll.

Meanwhile through the leadership agreement, the trial will proceed until sundown Friday, at which point it will pause for the Sabbath, and then resume on Sunday afternoon. The move accommodates one of Trump's lawyers, who is Jewish.

According to the rules, a majority vote will be needed if either side wants to call witnesses. Trump has already declined an invitation to testify.