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.
Here's a look at things to know about what's next for the former president and Congress.
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Why was Trump impeached again?
Then-President Donald Trump was impeached by the U.S. House for a historic second time on Jan. 13, charged with "incitement of insurrection" over the deadly mob siege of the U.S. Capitol just days earlier. The four-page article of impeachment says that Trump "gravely endangered the security of the United States and its institutions of Government."
The House voted 232-197 to impeach Trump, including ten Republicans. The proceedings moved at lightning speed, with lawmakers voting just one week after violent pro-Trump loyalists stormed the Capitol, egged on by the president's calls for them to "fight like hell" against the election results.
Trump's impeachment trial will be the first of a U.S. president no longer in office. While some have questioned impeaching the president so close to the end of his term, there is precedent. In 1876, during the Ulysses Grant administration, War Secretary William Belknap was impeached by the House the day he resigned, and the Senate convened a trial months later. He was acquitted.
When will Trump's second impeachment trial begin? How long will it last?
Opening arguments in the Senate impeachment trial will begin Tuesday.
It's unclear how long the trial will last. 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.
Who are the House impeachment managers and other major players involved?
The House impeachment managers are Rep. Jamie Raskin of Maryland, Reps. Diana DeGette and Joe Neguse of Colorado, Rep. David Cicilline of Rhode Island, Rep. Joaquin Castro of Texas, Reps. Eric Swalwell and Ted Lieu of California, Del. Stacey Plaskett of the U.S. Virgin Islands and Rep. Madeline Dean of Pennsylvania.
Democratic Sen. Patrick Leahy of Vermont will preside over the trial.
Defense lawyer David Schoen, a frequent television legal commentator, and Bruce Castor, a former district attorney in Pennsylvania who has faced criticism for his decision to not charge actor Bill Cosby in a sex crimes case, will helm Trump's legal defense team. They were named just one day after it was revealed that Trump had parted ways with an earlier set of attorneys with just over a week to go before his Senate trial.
Will the Senate convict Trump?
Senate Republicans voted overwhelmingly in late January against moving forward with the trial, making clear a conviction is unlikely.
In a 55-45 procedural vote, the Senate set aside an objection from Kentucky Sen. Rand Paul that would have declared the impeachment proceedings unconstitutional. Yet the support of 45 Republicans for declaring the trial invalid indicates that there are long odds for Trump's conviction, which would require the support of all Democrats and 17 Republicans, or two-thirds of the Senate.
Why impeach a former president?
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."
What will Trump's impeachment 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.
If convicted, will Trump be barred from running for president again in 2024?
If the Senate were to convict, lawmakers could then take a separate vote on whether to disqualify Trump from holding future office.
In the case of federal judges who were impeached and removed from office, the Senate has taken a second vote after conviction to determine whether to bar the person from ever holding federal office again.
Only a majority of senators would be needed to ban him from future office, unlike the two-thirds needed to convict.
What happened last year during Trump's first impeachment?
Trump was first impeached by the House in 2019 over his dealings with Ukraine. The trial in early 2020 lasted almost three weeks, but Trump was ultimately acquitted of trying to force Ukraine to investigate his then-Democratic challenger Joe Biden by withholding promised funding.
The Associated Press contributed to this report.