By Diane Cooke
Allegations that Donald Trump asked FBI Director James Comey to end an investigation into Michael Flynn's links to Russia have brought calls for the president to be impeached.
The new development, stemming from a memo written by Mr Comey, followed a week of controversy at the White House after Mr Trump fired the FBI director and then discussed sensitive national security information about Islamic State with Russian Foreign Minister Sergei Lavrov.
At least two congressional Democrats have demanded the president be impeached, but Democratic House leader Nancy Pelosi pushed back against those isolated calls.
However, the clamour grew in the wake of the latest claims. David Axelrod, Barack Obama's former senior advisor, tweeted: "I've been resistant to impeachment talk until now, but if Comey memo is true-and Comey is very credible - we are into a whole new deal here."
Trump wouldn't be the first president to be impeached. Bill Clinton (1998) and Andrew Johnson (1868) got there before him. Articles of impeachment were also passed against Richard Nixon by a congressional committee, but Nixon resigned before the House of Representatives could vote on the matter, meaning that technically he was not impeached.
Impeachment does not mean expulsion from office. Under the constitution, impeachment happens in the House of Representatives if a majority approves articles of impeachment previously approved in committee. Then impeachment goes to the Senate, where a two-thirds majority vote is required to convict the president, upon which he would be removed from office.
Both Johnson and Clinton were impeached in the House but then acquitted in the Senate and remained in office.
A president can be impeached for “Treason, bribery, or other high crimes and misdemeanors”, the constitution says. Needless to say, there’s debate over what all those terms mean.
Johnson was charged with breaking the law by removing the US secretary of war, which, in the aftermath of the civil war, was not his decision as president to make. Clinton was charged with obstruction of justice and with perjury, for allegedly lying under oath to a federal grand jury about his affair with Monica Lewinsky.
Had Nixon not resigned, he might have been convicted in the Senate on one of three charges: obstruction of justice, abuse of power or defiance of subpoenas. In any case, Gerald Ford, who was Nixon’s vice-president and who succeeded him, pardoned Nixon of any crimes a month after Nixon resigned.