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was "singularly responsible" for the deadly last month and acquitting the former president could damage American democracy, lawmakers leading the impeachment case said Tuesday, a week before his Senate trial begins.

Trump became the first president in history to be impeached twice when the House of Representatives charged him last month with inciting the mayhem inflicted by his followers when they invaded Congress on January 6.

In a pre-trial brief, the House impeachment managers made their case for the Senate to convict, saying the American people should be protected "against a president who provokes violence to subvert our democracy."

Trump's impeachment was triggered by a speech he delivered to a crowd on the National Mall just before the riot, telling them that Joe Biden had stolen the presidential election and that they needed to march on Congress and show "strength."

The mob stormed the Capitol, fatally wounded one police officer, wrecked furniture and forced terrified lawmakers to hide, interrupting a ceremony to put the legal stamp on Biden's victory.

The nine impeachment managers, all Democrats, argued in their sweeping 77-page document that Trump's speech had whipped the crowd into a "frenzy."

Trump, they said, "is singularly responsible for the violence and destruction" during the riot that left five people dead and threatened the lives of lawmakers and vice president Mike Pence.

"In a grievous betrayal of his oath of office, President Trump incited a violent mob to attack the United States Capitol," wrote the lawmakers, led by congressman Jamie Raskin.

"If provoking an insurrectionary riot against a joint session of Congress after losing an election is not an impeachable offense, it is hard to imagine what would be," the brief states.

Failure to convict Trump "would embolden future leaders to attempt to retain power by any and all means - and would suggest that there is no line a president cannot cross."

No 'January Exception'

Although Trump was impeached on January 13, his term ended a week later - before the beginning of the Senate trial.

"The present proceedings are moot and thus a nullity since the 45th president cannot be removed from an office he no longer occupies," Trump lawyers Bruce Castor and David Schoen wrote in their own brief outlining the case for the defense.

They also said Trump's speech in Washington, and his repeated refusal to accept the election results, amounted to protected free speech.

"The president exercised his First Amendment right under the Constitution to express his belief that the election results were suspect," the lawyers wrote.

Democrats rejected outright the reasoning that Trump cannot be tried once out of office.

"There is no 'January Exception' to impeachment or any other provision of the Constitution," they wrote, adding that a president must answer for his conduct in office "from his first day in office through his last."

They point to multiple videos - expected to be used as evidence in the trial - which they say show Trump inciting the crowd to commit violence, and show rioters chanting "Hang Mike Pence!" and hunting for House Speaker Nancy Pelosi.

Trump spent much of his time after the November 3 vote claiming that the election was stolen through massive fraud.

Dozens of courts in multiple states found the argument baseless.

But impeachment managers argued that Trump's constant promoting of the unfounded accusations that the election was stolen fueled his supporters into backing efforts to overturn the election.

When those efforts failed, the Democrats wrote, Trump "summoned a mob to Washington, exhorted them into a frenzy, and aimed them like a loaded cannon down Pennsylvania Avenue."

The Senate's 100 members take up the impeachment trial on February 9, and it is expected to last at least one week.

Democrats acknowledge that a conviction is unlikely. With the chamber evenly split 50-50, Democrats would need at least 17 Republicans to break with Trump in order to surpass the two-thirds threshold necessary for conviction.

Should that occur, a subsequent vote would be held, with a simple majority required to ban Trump from holding public office in the future.