The Senate will launch Donald Trump's historic second impeachment trial on Tuesday, with lawyers for the former president insisting he is not guilty of inciting mob violence at the Capitol to overturn the election.

Prosecutors, however, say he must be convicted of the "most grievous constitutional crime" even though he is gone from the White House.

Mr Trump faces a sole charge of incitement to insurrection over the January 6 Capitol siege, an attack that stunned the nation and the world after he encouraged a rally crowd to "fight like hell" for his presidency.

Rioters stormed the building trying to stop the certification of President-elect Joe Biden's victory, in an insurrection in which five people lost their lives.

No witnesses are expected to be called, in part because the senators sworn as jurors will be presented with graphic videos of the scenes they witnessed that day, when they were forced to flee for safety.

Under Covid-19 protocols, senators will distance for the trial, some even using the visitors' galleries. Holed up at his Mar-a-Lago club in Florida, Mr Trump has declined a request to testify.

The first president to face charges after leaving office and the first to be twice impeached for high crimes and misdemeanours, Mr Trump continues to challenge the nation's civic norms and traditions even in defeat.

Security remains extremely tight at the Capitol. While acquittal is likely, the trial will test the nation's attitude toward his brand of presidential power, the Democrats' resolve in pursuing him and the loyalty of Mr Trump's Republican allies defending him.

"In trying to make sense of a second Trump trial, the public should keep in mind that Donald Trump was the first president ever to refuse to accept his defeat," said Timothy Naftali, a clinical associate professor at New York University and an expert on Richard Nixon's impeachment saga.

"This trial is one way of having that difficult national conversation about the difference between dissent and insurrection."

White House press secretary Jen Psaki said on Monday that Mr Biden would be occupied with the the business of the presidency and would not spend much time watching the televised proceedings.

"He'll leave it to his former colleagues in the Senate," she said.

In trial filings on Monday, lawyers for the former president lobbed a wide-ranging attack against the House case, dismissing the trial as "political theatre" on the same Senate floor that was invaded by the mob.

Mr Trump's defenders are preparing to challenge both the constitutionality of the trial and any suggestion that he was to blame for the insurrection.

They suggest Mr Trump was simply exercising his First Amendment rights when he encouraged his supporters to protest at the Capitol, and they argue the Senate is not entitled to try Mr Trump now that he has left office.

"While never willing to allow a 'good crisis' to go to waste, the Democratic leadership is incapable of understanding that not everything can always be blamed on their political adversaries," the Trump lawyers say.

House impeachment managers filed their own document on Monday, asserting Mr Trump had "betrayed the American people" and there was no valid excuse or defence.

"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," the Democrats said.

The trial will begin on Tuesday with a debate and vote on whether it is constitutionally permissible to prosecute the former president, an argument that could resonate with Republicans keen on voting to acquit Mr Trump without being seen as condoning his behaviour.

Under an agreement between Senate Majority Leader Chuck Schumer and Republican leader Mitch McConnell, the opening arguments are set to begin on Wednesday, with up to 16 hours per side for presentations.

Mr Trump's second impeachment trial is expected to diverge from his first one last year. That was a more complicated affair, with Mr Trump charged with privately pressuring Ukraine to dig up dirt on Mr Biden.

This time, Mr Trump's "stop the steal" rally rhetoric and the storming of the Capitol played out for the world to see. The trial could be over in half the time.