Democrats, led by House impeachment manager Stacey Plaskett, have begun formally making their case that Donald Trump should be convicted for inciting the US Capitol siege, a day after a divided Senate concluded his impeachment trial could proceed though he's left office.

Nine Democratic House members acting as Trump's prosecutors began the proceedings on Tuesday by airing a graphic video that interspersed excerpts of Trump's speech with scenes from the attack, including clips of police officers under assault and a rioter fatally shot by authorities.

The House of Representatives has charged the former president with inciting an insurrection after he delivered a fiery speech on January 6 exhorting thousands of supporters to march on the US Capitol where members of Congress were gathered to certify President Joe Biden's electoral victory.

In an assault that stunned the world, rioters stormed the building in a futile effort to stop Biden's win, sending lawmakers into hiding and leaving five people dead, including a police officer.

Democrats say Capitol Police removed House Speaker Nancy Pelosi from the Capitol complex entirely because they feared for her safety on January 6.

Prosecutors at Donald Trump's second impeachment trial on Wednesday played audio of Pelosi's barricaded staffers whispering for help and showed images of the mob trying to break down a door into Pelosi's office.

The 80-year-old Pelosi was a longtime political target of the president, who derisively nicknamed her "Crazy Nancy".

Ms Plaskett says Pelosi was rushed to a secure off-site location because some of the rioters publicly declared their intent to harm or kill Pelosi.

Plaskett said if the rioters had found Pelosi, they would have killed her. She said: "They did it because Donald Trump sent them on this mission."

On Tuesday, the Senate voted largely along party lines that the impeachment trial could move ahead even though Trump's term ended on January 20. Six out of 50 Republican senators broke with their caucus to side with Democrats.

The outcome suggests Democrats face long odds in securing a conviction and barring Trump from ever again seeking public office. A two-thirds majority in the Senate must vote to convict, which means at least 17 Republicans would have to defy Trump's still-potent popularity among Republican voters.

The trial is unfolding inside the Senate chamber, where senators now serving as jurors were forced to flee for their safety a month ago as a mob broke into the building.

Nine Democratic House members acting as Trump's prosecutors began the proceedings on Tuesday by airing a graphic video that interspersed excerpts of Trump's speech with scenes from the attack, including clips of police officers under assault and a rioter fatally shot by authorities.

The Democrats accused Trump of committing an unforgivable offence by encouraging his backers to block the peaceful transfer of power, a hallmark of American democracy.

"If that's not an impeachable offence, then there's no such thing," said Representative Jamie Raskin, who delivered an emotional speech recounting how he became separated from his daughter and son-in-law during the violence.

Trump's lawyers argued that the former president's rhetoric, including repeated false claims that the election was stolen, is protected by the First Amendment's guarantee of free speech and that the individuals who breached the Capitol, not Trump, were responsible for their own criminal behaviour.

The lawyers sought to portray the trial as a sham, asserting that Democrats had weaponised impeachment to end Trump's political career while ignoring basic principles of fairness and due process.

"We are really here because the majority in the House of Representatives does not want to face Donald Trump as a political rival in the future," Bruce Castor, one of Trump's lawyers, told senators.

The Democratic-led House impeached Trump one week after the riot, making him only the third US president to be impeached and the first to be impeached twice.

Trump's first impeachment trial, which stemmed from his efforts to pressure Ukraine to investigate Biden during the presidential campaign, ended in an acquittal a year ago in what was then a Republican-controlled Senate.

Party leaders have agreed on a fast-moving schedule that could lead to a vote on conviction or acquittal by early next week. Some Democrats had expressed concern that a prolonged trial could delay progress on Biden's agenda, including a proposed $US1.9 trillion coronavirus relief package.

Australian Associated Press