WASHINGTON (AP) — Former President Donald Trump's historic second impeachment trial begins Tuesday, forcing the Senate to decide whether to convict him of incitement of insurrection after a violent mob of his supporters laid siege to the U.S. Capitol on Jan. 6.

While Trump’s acquittal is expected, all 100 senators will first have to sit at their desks and listen to hours of graphic testimony from House Democrats about the riots, which left five people dead. The House impeached Trump on Jan. 13, one week after the violence.

A look at the basics of the upcoming impeachment trial:

HOW DOES THE TRIAL WORK?

The Constitution says the House has the sole power of impeachment while the Senate has the sole power to try the individual on the charges. The person being impeached — who can be the president, the vice president or any civil officer of the United States — can be convicted by two-thirds of the senators present.

The House appoints managers as prosecutors who set up on the Senate floor, along with the defendant’s lawyers, to present their case. The prosecutors and Trump's defense team will have a set amount of time to make arguments, and then senators can ask questions in writing before a final vote.

The chief justice of the United States normally presides over the trial of a president, but because Trump has left office, the presiding officer will be Sen. Patrick Leahy, D-Vt., who is the ceremonial head of the Senate as the longest-serving member of the majority party.

Once the senators reach a final vote on the impeachment charge — this time there is just one, incitement of insurrection — each lawmaker will stand up and cast their vote: guilty or not guilty.

HOW LONG WILL THE TRIAL LAST?

Unclear. The Senate has to agree to the rules of the trial, and party leaders are still working out the details.

Trump's first impeachment trial, in which he was acquitted on charges that he abused power by pressuring Ukraine to investigate now-President Joe Biden, lasted almost three weeks. But this one is expected to be shorter, as the case is less complicated and the senators know many of the details already, having been in the Capitol during the insurrection.

And while the Democrats want to ensure they have enough time to make their case, they do not want to tie up the Senate for long. The Senate cannot confirm Biden's Cabinet nominees and move forward with their legislative priorities, such as COVID-19 relief, until the trial is complete.

WHY TRY TRUMP WHEN HE IS OUT OF OFFICE?

Republicans and Trump’s lawyers argue that the trial is unnecessary, and even unconstitutional, because Trump is no longer president and cannot be removed from office. Democrats disagree, pointing to opinions of many legal scholars and the impeachment of a former secretary of war, William Belknap, who resigned in 1876 just hours before he was impeached over a kickback scheme.

While Belknap was eventually acquitted, the Senate held a full trial. And this time, the House impeached Trump while he was still president, seven days before Biden’s inauguration.

If Trump were convicted, the Senate could take a second vote to bar him from holding office again. Democrats feel that would be an appropriate punishment after he told the angry mob of his supporters to “fight like hell” to overturn his election defeat.

Democrats also argue that there should not be a “January exception” for presidents who commit impeachable offenses just before they leave office. They say the trial is necessary not only to hold Trump properly accountable but also so they can deal with what happened and move forward.

“You cannot go forward until you have justice,” said House Speaker Nancy Pelosi this week. “If we were not to follow up with this, we might as well remove any penalty from the Constitution of impeachment.”

HOW IS THIS TRIAL DIFFERENT FROM TRUMP’S FIRST TRIAL?

Trump’s first trial was based on evidence uncovered over several months by the House about a private phone call between Trump and the president of Ukraine, as well as closed-door meetings that happened before and afterward. Democrats held a lengthy investigation and then compiled a report of their findings.

In contrast, the second trial will be based almost entirely on the visceral experience of a riot that targeted the senators themselves, in the Capitol building. The insurrectionists even breached the Senate chamber, where the trial will be held.

The fresh memories of Jan. 6 could make it easier for the House impeachment managers to make their case, but it doesn't mean the outcome will be any different. Trump was acquitted in his first trial a year ago Friday with only one Republican, Utah Sen. Mitt Romney, voting to convict, and there may not be many more guilty votes this time around.

In a test vote Jan. 26, only five Senate Republicans voted against an effort to dismiss the trial — an early indication that Trump is likely to be acquitted again.

WHAT WILL TRUMP'S LAWYERS ARGUE?

Beyond the constitutionality of the trial, Trump’s lawyers say that he did not incite his supporters to violence and that he did nothing wrong. “It is denied that President Trump ever endangered the security of the United States and its institutions of Government,” they wrote in a brief for the trial. “It is denied he threatened the integrity of the democratic system, interfered with the peaceful transition of power, and imperiled a coequal branch Government.”

Trump's lawyers also say he was protected by the First Amendment to “express his belief that the election results were suspect.”

There was no widespread fraud in the election, as Trump claimed falsely over several months and again to his supporters just before the insurrection. Election officials across the country, and even former Attorney General William Barr, contradicted his claims, and dozens of legal challenges to the election put forth by Trump and his allies were dismissed.

WHAT WOULD ACQUITTAL MEAN FOR TRUMP?

A second impeachment acquittal by the Senate would be a victory for Trump — and would prove he retains considerable sway over his party, despite his efforts to subvert democracy and widespread condemnation from his GOP colleagues after Jan. 6.

Still, acquittal may not be the end of attempts to hold him accountable. Sens. Tim Kaine, D-Va., and Susan Collins, R-Maine, floated a censure resolution after last month’s vote made clear that Trump was unlikely to be convicted.

While they haven’t said yet if they will push for a censure vote after the impeachment trial, Kaine said this week that “the idea is out there on the table and it may become a useful idea down the road.”

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Associated Press writers Eric Tucker and Lisa Mascaro contributed to this report.