Donald Trump's legal team argued Monday on the eve of his impeachment trial that the case is constitutionally flawed and it was "absurd" to hold the former president responsible for the US Capitol riot.

The House of Representatives impeached Trump for a historic second time last month over his role in inciting the deadly storming of Congress by his supporters, and his trial begins Tuesday with the Senate's 100 members sitting as jurors.

For only the fourth time ever an impeached American president is on trial, and two of these trials have involved Trump -- a deeply damaged political figure who despite that remains a major force in the Republican Party.

Charged with "incitement of insurrection," he is likely once again to avoid conviction due to loyal party 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 defense 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 somehow conjured up a mob to commit violent crime, and that those who attacked the Capitol on January 6 did so on their own.

The lawyers were equally forceful in swatting away the viability of trying a former president.

"The Senate is being asked to do something patently ridiculous: try a private citizen in a process that is designed to remove him from an office that he no longer holds."

'Leave it to the Senate'

President Joe Biden, who succeeded Trump on January 20, weighed in briefly as he returned to the White House Monday after a weekend in Delaware, 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 assured reporters Biden's views on Trump were clear, and that he ran against the president "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 said.

The congressional Democrats prosecuting the trial, known as House impeachment managers, were due to provide their final counterarguments later Monday before the court of impeachment is called to order the following day.

The Senate's 100 members will sit in judgment of Trump beginning 1:00 pm (1800 GMT) on Tuesday in sessions that will air live across the nation.

They are stepping into uncharted territory, as no president has endured an impeachment trial after already leaving office.

Trump's defense team says the House of Representatives, which impeached him on January 13, just one week after the riot, "deprived the 45th President of due process" by failing to conduct committee hearings that would consider evidence in the case.

- Loyal conservatives -

The proceedings will take place in the 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 pre-trial brief that Trump, who rallied supporters in Washington shortly before the mob assault, was "singularly responsible" for the unrest which left five people dead including a US Capitol Police officer.

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 who may eye 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.

But the president retains loyalty among a branch of conservatives in Congress.

Conservative congressman Andy Biggs called the impeachment process "illegitimate" in a video on Monday, calling it a "second bogus impeachment by the Democrats who want to erase the legacy of president Donald J Trump."