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The House of Representatives has charged Trump, a 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 an narrowly divided Senate, said on Wednesday the incitement started long before January 6.
"Trump realized 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."
Representative Joaquin Castro cited what he called blatant acts of political intimidation against election workers in states Trump was losing. In Philadelphia, Atlanta and Milwaukee, Castro said, Trump's supporters tried to use armed force to disrupt the counting of votes.
“They believed it was their duty to quite literally fight to stop the count,” Castro said.
Trump's actions threatened a hallmark of American democracy, the peaceful transfer of power, the Democratic managers said.
"This case is not about blaming an innocent bystander for the horrific violence and harm that took place on January 6," lead manager Jamie Raskin said as he opened the proceedings.
"This is about holding accountable the person singularly responsible for inciting the attack."
Democrats face long odds to secure a conviction, which could lead to a vote barring 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 Trump's still-potent popularity among Republican voters.
Republican Senator Ron Johnson, a Trump ally, told reporters at a break that the proceedings would become "pretty tedious."
"This is a political exercise," he said.
On Tuesday, just six out of 50 Republican senators broke with their caucus to vote that the trial could move ahead even though Trump's term ended on January 20.
In an Ipsos poll for Reuters that was released on Wednesday, 47% of respondents said Trump should be convicted while 40% said he should not, with opinions split along party lines.
The trial in the Senate is not the only probe Trump faces after leaving the White House and losing the presidential protections that shielded him from prosecution.
Prosecutors in Georgia’s biggest county have opened a criminal investigation into Trump’s attempts to influence the state’s election results after he was recorded in a Jan. 2 phone call pressuring the Secretary of State to "find" enough votes to overturn his Georgia loss.
Trump's lawyers, who will have 16 hours to deliver their side of the argument after the House managers are finished, 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 were responsible for their own criminal behaviour.
They also say the trial is politically motivated.
"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 on Tuesday.
It was still not clear whether either side would present witnesses, but Democratic Senator Dick Durbin told CNN on Wednesday: "We have plenty of witnesses. We have 100 witnesses sitting in the chairs and the senators."
The Senate trial could conclude as early as Saturday or Sunday, according to a senior Senate aide.
Some Democrats had expressed concern that a prolonged trial could delay progress on Biden's agenda, including a proposed $US1.9 trillion ($NZ2.6 trillion) coronavirus relief package.
Biden was not planning to watch the trial, the White House said. Asked on Tuesday 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.
Trump is the first US president to be impeached twice. His first impeachment trial, which stemmed from his efforts to pressure Ukraine to investigate 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.