Maryland Democratic Rep. Jamie Raskin will lead the House's prosecution, tapped by Speaker Nancy Pelosi to serve as the top impeachment manager. Raskin, a Harvard-educated, former constitutional law professor serving in his third term in the House, was the lead author of the impeachment article in the wake of the  Jan. 6 insurrection at the U.S. Capitol.

Days earlier, on New Year's Eve, Raskin's son Thomas, who was following his father's footsteps at Harvard Law School, took his own life after battling depression. The Maryland lawmaker drew a standing ovation from his colleagues on the House floor as he thanked them for their "love and tenderness" over the loss of his son. He then proceeded with his remarks on the objections over accepting the election results. Less than an hour later, the Capitol was under siege and Raskin was sheltering under a desk with one of his two daughters and his son-in-law.

-ABC News' John Parkinson

Trump will be the first president to undergo an impeachment trial after leaving office, but opponents of the impeachment say a trial may be unconstitutional.

The public should expect to see four attorneys on the Trump team during the trial: David Schoen, Bruce Castor, Michael van der Veen and Julieanne Bateman.

The newly appointed legal team submitted its first legal brief on Feb. 2, arguing the trial is unconstitutional because Trump is no longer in office. The trial's legality has been called into question since the beginning but the Senate voted to proceed.

The brief also argues that Trump's use of social media and comments made on Jan. 6 are protected by the First Amendment. There is a possibility that Trump's defense may skirt into claims of election fraud, despite Trump and his allies losing dozens of court cases on the issue and the 2020 election results being certified by Congress.

Trump's lawyers filed another brief on Feb. 8, further elaborating their argument against the trial's constitutionality and asking the Senate to dismiss the charges. House impeachment managers followed with a five-page response to Trump's legal team.

-ABC News' Katherine Faulders and Tia Humphries

Democrats are preparing to argue that Trump constituted the "most grievous constitutional crime ever committed by a president" and is "singularly responsible" for the deadly riot at the Capitol on Jan. 6, and that the Senate can't establish "a January exception to the Constitution," according to senior aides on the impeachment managers' team.

The managers, led by Rep. Jamie Raskin, D-Md., have been meeting every day -- sometimes twice a day -- since they were named to work on the case, mostly virtually given the pandemic. 

They promised a "succinct and to the point and non-repetitive" argument laying out how the attack happened in "plain sight" and left behind "overwhelming evidence."

"This is not about politics," the aides said, adding that they won't touch any senators' support of Trump.

"This is personal for them. They experienced the attack, their staff experienced the attack," one aide said. "They're not taking this lightly, they find no joy in this."

On the constitutional question of trying a former president, aides said, "This will not be like a constitutional convention," and likened it instead to a "violent criminal prosecution."

They called the argument that the trial is unconstitutional "just not common sense." 

"It is unthinkable that the framers would say that that a president could not be impeached, no matter what he or she did in the final days of office would allow the president to misuse power at the most dangerous time right when a president wants to hold on to power, that the president can do whatever that president wants without fear of losing office or be barred from running again. That cannot be," one aide said.

-ABC News Congressional Corespondent Rachel Scott, Katherine Faulders, Benjamin Siegel, Trish Turner and Allison Pecorin

The Senate is taking a 15-minute break following the start of opening arguments from the House impeachment managers.