Washington (AFP) - The past week has seen extraordinary political turmoil in Washington, as President Donald Trump faces grave accusations that he has obstructed justice.
With the White House running damage control, and critics smelling blood, some Democrats have mentioned the "I" word -- impeachment -- as a possibility for the 45th president, although initiating the procedure remains a hypothetical.
The Republican leader faces allegations he asked his FBI director James Comey to halt a probe of a senior former advisor, and dismissed Comey in a bid to curb an investigation into his campaign team's possible collusion with Russia.
No US president has been ousted from office under impeachment proceedings. Two were impeached by the House of Representatives but ultimately acquitted in the US Senate: Andrew Johnson in 1868 and Bill Clinton in 1998.
Richard Nixon resigned in 1974 to avoid certain impeachment by Congress over the Watergate scandal.
Here is a look at the rarely used tool to oust a sitting US president from power.
- How does it work? -
Impeachment is a two-step process. If lawmakers believe a president is guilty of what the US Constitution calls "treason, bribery, or other high crimes and misdemeanors," they may initiate articles of impeachment.
The first vote takes place in the House, where a simple majority is needed.
In the event of impeachment, the case shifts to debate in the US Senate, where a two-thirds majority is needed for conviction in the 100-member chamber.
If that threshold is reached, the president is ousted -- with no recourse for appeal.
Should the Senate fall short of conviction, the president is acquitted and remains in office.
- What's the role of the courts? -
They don't have one.
"There isn't any judicial review of impeachment decisions, so Congress just needs to be satisfied that Trump committed high crimes or misdemeanors," Jens David Ohlin, a law professor and associate dean at Cornell Law School told AFP.
"They are the ultimate judge of what meets that standard."
Impeachment therefore is at the crossroads of politics and the law. "There's no requirement that the president must have been indicted" for a crime, Ohlin added.
- How far along are we? -
The process hasn't started -- and may never start.
Two Democrats in the House of Representatives, Maxine Waters and Al Green, have called for articles of impeachment to be drawn up, but most lawmakers are holding fire, fearing it could transform the process into a purely partisan exercise.
Senior Democrats say it is too early to call for impeachment, and that the facts of the case must first be established.
"It cannot be perceived as an effort to nullify the election by other means," said Adam Schiff, the top Democrat on the House Intelligence Committee.
Several lawmakers point out however that obstruction of justice is an impeachable offense. It was Nixon's downfall during Watergate, and led to Clinton's impeachment in the House.
The latest allegation suggesting obstruction of justice is the report that in February Trump asked Comey to back away from the probe into his former national security advisor, Michael Flynn. Comey is said to have written a memo about the meeting during which Trump allegedly made the request.
Justin Amash, a Republican who has been critical of Trump in the past, agreed that if the Comey memo proves to be true, it would constitute grounds for impeachment. He was the first Republican lawmaker to raise the possibility publicly. "But everybody gets a fair trial in this country," he added.
For that reason, members of Congress are impatiently waiting for Comey to accept their invitation to testify before them, and give his version of events for the first time.