Former President Donald Trump faces an unprecedented second impeachment trial this week. Adding to the historic nature of the proceeding is that he is no longer in office and the members of the Senate who will decide his fate are among the victims in the Capitol siege, which he is accused of instigating.
The House of Representatives voted 232-197 on Jan. 13 to impeach Trump for an unprecedented second time for his role in the Jan. 6 riot and breach of the Capitol, which occurred as a joint session of Congress was ratifying the election of President Biden.
The extraordinary step of a second impeachment, which charged Trump with incitement of insurrection, took place just days before Trump was set to leave office. Only two other presidents -- Andrew Johnson and Bill Clinton -- have been impeached and none have been convicted.
Unlike Trump’s first impeachment in 2019 (in which no Republican voted to impeach), 10 members of the House GOP, including conference chair Liz Cheney, R-Wyo., voted for impeachment and denounced the president’s actions. Democratic House impeachment managers argued in a brief ahead of his trial, which starts in earnest Feb. 9, that Trump bore "unmistakable" responsibility for the siege and called it a "betrayal of historic proportions."
"He summoned a mob to Washington, exhorted them into a frenzy, and aimed them like a loaded cannon down Pennsylvania Avenue," the managers wrote.
While some Republicans have spoken out against Trump's rhetoric in the wake of the siege, it is unlikely that the former president will be convicted because it would require at least 17 Republican Senators and all 50 Democrats to agree. Some GOP members have questioned the constitutionality of trying a former president.
Indeed, that's the argument that Trump's lawyers made in their own brief ahead of the trial, calling the proceeding a "legal nullity" and leaving the door open to argue the very claims of election fraud that some say sparked the riot.
"It is admitted that President Trump addressed a crowd at the Capitol ellipse on January 6, 2021 as is his right under the First Amendment to the Constitution and expressed his opinion that the election results were suspect, as is contained in the full recording of the speech," the president's lawyers wrote. The lawyers denied that Trump participated in insurrection.
Meanwhile, last week, some 144 constitutional law scholars published a letter in The New York Times, calling a defense based on the First Amendment “legally frivolous.”
Here's how the impeachment process works:
The presidential impeachment process
An impeachment proceeding is the formal process by which a sitting president of the United States is accused of wrongdoing. It is a political process and not a criminal process.
The articles of impeachment (in this case there's just one) are the list of charges drafted against the president. The vice president and all civil officers of the U.S. can also face impeachment.
The process begins in the House of Representatives, where any member may make a suggestion to launch an impeachment proceeding. It is really up to the speaker of the House in practice, to determine whether or not to proceed with an inquiry into the alleged wrongdoing, though any member can force a vote to impeach.
Over 210 House Democrats introduced the most recent article of impeachment on Jan. 11, 2021, contending Trump "demonstrated that he will remain a threat to national security, democracy and the Constitution if allowed to remain in office and has acted in a manner grossly incompatible with self-governance and the rule of law."
The impeachment article, which seeks to bar Trump from holding office again, also cited Trump's controversial call with the Georgia Republican secretary of state where he urged him to "find" enough votes for Trump to win the state and his efforts to "subvert and obstruct" certification of the vote.
And it cited the Constitution's 14th Amendment, noting that it "prohibits any person who has 'engaged in insurrection or rebellion against' the United States" from holding office.
House Speaker Nancy Pelosi and other Democrats accelerated the procedure -- not holding any hearings -- and voted just a week before the inauguration of President Biden.
The vote requires a simple majority vote, which is 50% plus one (218), after which the president is impeached.
Trump now faces a trial on the article in the Senate.
Justification for impeachment
When it comes to impeachment, the Constitution lists "treason, bribery, or other high crimes and misdemeanors," as justification for the proceedings, but the vagueness of the third option has caused problems in the past.
"It was a central issue with Andrew Johnson, and there was a question during Clinton's proceedings about whether his lie [to a federal grand jury] was a ‘low’ crime or a ‘high’ crime," Michael Gerhardt, a constitutional law professor at the University of North Carolina who authored a book on the impeachment process, told ABC News.
According to Suzanna Sherry, a law professor at Vanderbilt University who specializes in constitutional law, "nobody knows" what is specifically included or not included in the Constitution’s broad definition of “high crimes and misdemeanors.”
"It’s only happened twice and so the general thought is that it means whatever the House and the Senate think it means," Sherry said before Trump's first impeachment, and even if the House approves the article or articles of impeachment, the senators can choose to vote against the articles if they feel they are not appropriate.
Where does the Senate come in?
The Senate is tasked with handling the impeachment trial, which is presided over by the chief justice of the United States in the case of sitting presidents. However, in this unusual case, since Trump is not a sitting president, the largely ceremonial task has been left to the Senate pro tempore, Sen. Patrick Leahy, D-Vt., the chamber's most senior member of the majority party.
"The president pro tempore has historically presided over Senate impeachment trials of non-presidents," Leahy said in a statement in January. "When presiding over an impeachment trial, the president pro tempore takes an additional special oath to do impartial justice according to the Constitution and the laws. It is an oath that I take extraordinarily seriously."
To remove a president from office, two-thirds of the members must vote in favor – at present 67 if all 100 senators are present and voting.
If the Senate fails to convict, a president is considered impeached but is not removed, as was the case with both Clinton in 1998 and Andrew Johnson in 1868. In Johnson’s case, the Senate fell one vote short of removing him from office on all three counts.
In this trial, since the president has already left office, the real punishment would come if the president were to be convicted, when the Senate would be expected to vote on a motion to ban the former president from ever holding federal office again.
While the Senate trial has the power to oust a president from office, and ban him or her from running for future office, it does not have the power to send a president to jail. Disqualification from holding office, a separate process, requires a simple majority vote, according to the Congressional Research Service.
"The worst that can happen is that he is removed from office, that's the sole punishment," Sherry said of sitting presidents.
Trump's lawyers argued in their brief ahead of the second trial that the Senate cannot bar Trump from holding office in the future under the 14th Amendment because removal is a precondition for disqualification and as a private citizen the body has no jurisdiction over him.
That said, a president can face criminal charges at a later point. Sherry points out that in the Constitution "the party convicted shall nevertheless be liable and subject to indictment, trial, judgment and punishment, according to law."
In a case in which a president was actually removed from office, the vice president would assume office under the 25th Amendment, which was ratified in 1967. Then the new president would nominate a new vice president who would have to be confirmed by a majority of both houses of Congress.
What does an impeachment vote mean for a sitting president and for a former president?
A president can continue governing even after he or she has been impeached by the House of Representatives.
Past presidential impeachments
The House voted to impeach Trump on Dec. 18, 2019, on two articles of impeachment, one for abuse of power and one for obstruction of justice, in connection with his alleged quid pro quo call with the Ukrainian president.
Following a three-week trial, the Republican controlled Senate acquitted Trump on Feb. 5, 2020, with just one Republican -- Mitt Romney of Utah -- voting to convict.
Johnson faced impeachment in 1868 after clashing with the Republican-led House over the “rights of those who had been freed from slavery,” although firing his secretary of war, Edwin Stanton, who was backed by the Republicans, led to the impeachment effort. The articles of impeachment centered on the Stanton event, according to the Senate.
Clinton, whose impeachment was connected to the cover-up of his affair with White House intern Monica Lewinsky while in office, was 22 votes away from reaching the necessary number of votes to convict in the Senate.
Richard Nixon faced three articles of impeachment related to the Watergate scandal, in which he allegedly obstructed the investigation and helped cover up the crimes surrounding the break-in.
But he didn’t let the process get any further, resigning before the House could impeach him.
Editor's Note: This story was originally published in 2017 and has been updated periodically.