LIVE: Oral arguments set to kick off in Trump's Senate impeachment trial over the Capitol siege

The order of business on Tuesday was to hold a debate on the constitutionality of having an impeachment trial for Trump in the first place, given that he's no longer in office.

In a previous motion on the matter, five Republican senators — Lisa Murkowski, Susan Collins, Mitt Romney, Pat Toomey, and Ben Sasse — broke ranks and voted with their Democratic colleagues to declare Trump's trial constitutional, in a vote of 55 to 45.

Following Tuesday's debate, another Republican senator, Bill Cassidy of Louisiana, defected and joined his five colleagues in determining that Trump's trial doesn't run afoul of the Constitution. The final vote was 56 to 44.

• The House managers' argument: The impeachment managers say there is no "January exception" to impeachment because it would mean presidents could act with immunity during their final days in office. Trump's actions are impeachable, they said, because he undertook them while in office. Additionally, removal from office is not the only objective of impeachment because being barred from holding office in the future is also a possibility.
• The defense's argument: Trump's defense lawyers argued that even holding a trial was unconstitutional because Trump was no longer in office and therefore could not be removed via an impeachment trial. They also argued that Trump was deprived of due process and that the Senate was not the appropriate jurisdiction to "try" Trump.
• Rep. Jamie Raskin teared up recounting being trapped in the Capitol: Raskin, the lead impeachment manager, gave an emotional speech on the Senate floor where he recalled what it was like being in the Capitol during the siege with his daughter and his son-in-law. It was one day after Raskin and his family buried his son, Tommy, who died by suicide on New Year's Eve.
  • He choked up as he described his "kids hiding under the desk, placing what they thought were their final texts and whispered phone calls to say their goodbyes." He continued: "They thought they were going to die."
• Trump's lawyer was brutally mocked for a long and meandering opening statement: Bruce Castor Jr.'s lengthy, rambling statement raised eyebrows across the internet as lawyers, constitutional experts, and members of the public questioned where he was going. Several Republican senators also slammed Trump's defense team after the proceedings, calling it "disorganized" and "terrible," and saying they were "stunned" and "perplexed" by the arguments.

Trump is the only president in US history to have been impeached twice.

The first time, Trump was charged with abuse of power and obstruction of Congress in connection to the Ukraine scandal. This time, he faces a single article of impeachment accusing him of "incitement of insurrection" related to the deadly Capitol siege on January 6.

There's also a looming question of constitutionality this time around, as several Republicans as well as Trump's defense team have argued that Trump cannot be tried and removed from office now that he's no longer president.

The mechanics of this trial are also slightly different.

According to the US Constitution, the chief justice of the Supreme Court presides over a president's impeachment trial. But there's no playbook on who presides over the trial of a former president.

For Trump's second impeachment trial, Vermont Sen. Patrick Leahy, who is the president pro tempore of the Senate — the longest-serving Democrat in the chamber — will preside. Leahy was also in the Capitol the day of the siege, meaning he has the unique role of serving as judge, juror, and witness in this impeachment trial — a point Trump's lawyer, David Schoen, raised during Tuesday's debate over constitutionality and due process.