US Democrats have argued Donald Trump planted the seeds for the deadly attack on the Capitol long before 6 January with false claims the election was stolen, and said lawmakers had an obligation to hold the former president accountable.
The House of Representatives has charged the Republican with inciting an insurrection by exhorting thousands of supporters to march on the Capitol on the day Congress was gathered to certify Democrat Joe Biden's electoral victory.
The rioters stormed the building, sending lawmakers into hiding and leaving five people dead, including a police officer.
The nine Democratic House managers prosecuting the case for impeachment, an uphill task in a narrowly divided Senate, said on Wednesday the incitement started long before 6 January.
"Trump realised last spring that he could lose the November election and began planting seeds of anger among his supporters by saying he could lose only if it was stolen," said Representative Joseph Neguse.
"If we are to protect our republic and prevent something like this from ever happening again, he must be convicted."
The Democratic managers said Mr Trump threatened a hallmark of American democracy, the peaceful transfer of power.
"This case is not about blaming an innocent bystander for the horrific violence and harm that took place on Jan 6," Representative Jamie Raskin said as he opened the proceedings.
"It will show that Donald Trump surrendered his role as commander-in-chief and became the inciter-in-chief of a dangerous insurrection."
Democrats face long odds to secure a conviction, which could lead to a vote barring Mr Trump from seeking public office again.
A two-thirds majority in the Senate must vote to convict, which means at least 17 Republicans would have to defy the former president's still-potent popularity among Republican voters.
On Tuesday, the Senate voted that the trial could move ahead even though Mr Trump's term ended on 20 January.
Six out of 50 Republican senators broke with their caucus to side with Democrats.
The impeachment is not the only probe into Mr Trump's behaviour.
Prosecutors in one Georgia county are investigating Mr Trump's attempts to overturn the state's election results, according to a letter seen by Reuters.
On Monday, the office of Georgia's Republican Secretary of State Brad Raffensperger opened its own probe into a call made by Mr Trump on 2 January pressuring him to "find" more votes.
First amendment
Donald Trump's lawyers argue that the former president's rhetoric is protected by the First Amendment's guarantee of free speech and that the individuals who breached the Capitol, not Mr Trump, were responsible for their own criminal behaviour.
They also say the trial is only aimed at ending Mr Trump's political career.
"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 his lawyers, told senators on Tuesday.
Attorney Bruce Castor walks through The Senate Reception Room ahead of the second day of Donald Trump's second impeachment trial. Source: The New York Times
Party leaders have agreed on a fast-moving schedule that could lead to a vote on conviction or acquittal by early next week.
It was still not clear whether either side would present witnesses.
Democratic Senator Richard Durbin told CNN on Wednesday, "We have plenty of witnesses. We have 100 witnesses sitting in the chairs and the senators."
Some Democrats had expressed concern that a prolonged trial could delay progress on Joe Biden's agenda, including a proposed $US1.9 trillion ($A2.46 trillion) coronavirus relief package.
Mr Biden was not planning to watch the trial, the White House said, and spent Tuesday meeting business leaders in the Oval Office.
Asked about the proceedings, the president said he was focused on his own job.
"The Senate has their job; they're about to begin it. I'm sure they're going to conduct themselves well," he said.
Donald Trump is the first US president to be impeached twice.
His first impeachment trial, which stemmed from his efforts to pressure Ukraine to investigate Joe Biden, ended in an acquittal a year ago in what was then a Republican-controlled Senate.
No US president has ever been removed from office via impeachment.
Bill Clinton was impeached in 1998 and Andrew Johnson in 1868 but they also were acquitted.
Richard Nixon resigned in 1974 rather than face impeachment over the Watergate scandal.
