WASHINGTON (AP) — Donald Trump’s second impeachment trial is starting, an undertaking like no other in U.S. history. The defeated former president stands charged by the House with inciting the deadly mob attack on the Capitol to overturn the election in what prosecutors argue is the “most grievous constitutional crime.”

Trump's lawyers are insisting as the Senate trial opens Tuesday that he is not guilty of the sole charge of “incitement of insurrection,” his fiery words just a figure of speech as he encouraged a rally crowd to “fight like hell” for his presidency. The Capitol siege on Jan. 6 stunned the world as rioters stormed the building to try to stop the certification of President-elect Joe Biden’s victory. Five people died.

With senators gathered as the court of impeachment, the trial will begin with debate and then a vote on whether it’s constitutionally permissible to prosecute the former president, an argument that could resonate with Republicans keen on voting to acquit Trump without being seen as condoning his behavior. The House prosecutors will argue there is no “January exception” to a president's actions just before he leaves office and that the trial has ample precedent, according to aides familiar with the arguments and granted anonymity to discuss them ahead of the trial.

It appears unlikely that the House prosecutors will call witnesses, in part because the senators sworn as jurors, forced to flee for safety, will be presented with graphic videos recorded that day. At his Mar-a-Lago club in Florida, 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 misdemeanors while in office, Trump remains a challenge to the nation’s civic norms and traditions even in defeat. Security remains extremely tight at the Capitol.

Acquittal is likely, but the trial will test the nation’s attitude toward his brand of presidential power, the Democrats’ resolve in pursuing him, and the loyalty of 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, which ended with Nixon's resignation rather than impeachment.

“This trial is one way of having that difficult national conversation about the difference between dissent and insurrection,” Naftali said.

White House press secretary Jen Psaki said Monday that Biden will be busy with the business of the presidency and won't spend much time watching the televised proceedings. “He’ll leave it to his former colleagues in the Senate,” she said.

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

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 that 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 Trump now that he has left office.

House impeachment managers, in their own filings, asserted that Trump had “betrayed the American people” and there is no valid excuse or defense.

“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.

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

The House managers will argue the case like a “violent crime prosecution,” according to one of the aides familiar with their preparation, telling a succinct story beginning with Trump's false claims the election was stolen and ending with the attempt at insurrection on Jan. 6. They will show videos of the violence and lay out the day's events in a personal way, since they were themselves fleeing the rioters as it happened.

Trump's defense team has said it plans to counter with its own cache of videos of Democratic politicians making fiery speeches

After that there are hours for deliberations, witnesses and closing arguments. The trial is expected to continue into the weekend and next week.

A presidential impeachment trial has been conducted only three times before, leading to acquittals for Andrew Johnson, Bill Clinton and then Trump last year.

Typically senators sit at their desks for such occasions, but the COVID-19 crisis has upended even this tradition. Instead, senators will be allowed to spread out, in the “marble room” just off the Senate floor, where proceedings will be shown on TV, and in the public galleries above the chamber, to accommodate social distancing, according to a person familiar with the discussions.

Trump's second impeachment trial is expected to diverge from the lengthy, complicated affair of a year ago. In that case, Trump was charged with having privately pressured Ukraine to dig up dirt on Biden, then a Democratic rival for the presidency.

This time, 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.

The Democratic-led House impeached the president swiftly, one week after the most violent attack on Congress in more than 200 years. Five people died, including a woman shot by police inside the building and a police officer who died the next day of his injuries.

Initially repulsed by the graphic images of the attack, a number of Republican senators have cooled their criticism as the intervening weeks have provided some distance.

Senators were sworn in as jurors late last month, shortly after Biden was inaugurated, but the trial was delayed as Democrats focused on confirming the new president's initial Cabinet picks and Republicans sought to stall.

At the time, Sen. Rand Paul of Kentucky forced a vote to set aside the trial as unconstitutional because Trump is no longer in office. The 45 Republican votes in favor of Paul's measure suggest the near impossibility of reaching a conviction in a Senate where Democrats hold 50 seats but a two-thirds vote — or 67 senators — would be needed to convict Trump.

Only five Republicans joined with Democrats to reject Paul’s motion: Mitt Romney of Utah, Ben Sasse of Nebraska, Susan Collins of Maine, Lisa Murkowski of Alaska and Pat Toomey of Pennsylvania.

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Associated Press writer Hope Yen contributed to this report.