Here are five things to know about how the impeachment process works.
1) What sorts of offences trigger impeachment proceedings?
There is no hard-and-fast list. The House of Representatives decides. The Constitution says that presidents, vice-presidents and other federal officials can be impeached for "Treason, Bribery, or other high Crimes and Misdemeanours."
But what are "high Crime and Misdemeanours?" The document doesn't say. In the past, the House - where impeachment proceedings must begin - has defined those terms to mean something broader than just "federal crimes".
The House has also impeached presidents for behaviour that undermines the constitutional system or that brings shame to the office of president, regardless of whether that behaviour was criminal.
For instance: President Andrew Johnson, who was the first president to be impeached, was charged with firing one of his Cabinet members - in defiance of a law that said he needed the Senate's permission. He was also charged with, in essence, insulting Congress. One article of impeachment accused Johnson of "scandalous harangues" about legislators, made "with a loud voice".
2) How does impeachment work?
The House would vote on articles of impeachment, which are individual statements of offence. All it takes is a simple majority. If any of them pass, the President has been "impeached" - something like being indicted in a legal procedure.
Next, the President's case would move to the Senate, which acts as a 100-member jury. The House appoints "managers," who act like prosecutors, laying out the case for the President's removal. The chief justice of the United States presides over the proceedings if the President is on trial.
Convicting the President requires two-thirds of all senators to agree. If that happens, the President is automatically removed from office.
3) Has that ever happened?
Not to a president. Johnson, who was the first president to be impeached, escaped conviction by one vote in 1868. Bill Clinton was the second: The House brought impeachment proceedings against him in 1998, alleging perjury and obstruction of an investigation. The Senate acquitted him by a wider margin.
President Richard Nixon resigned in 1974 before the full House could vote on impeachment charges against him.
Beyond the cases that involve presidents, impeachment has been a tool rarely used in US history. Since 1789, only eight federal officials have been convicted by the Senate and removed from office. All eight were federal judges.
That list includes one current member of Congress: Representative Alcee Hastings, D, a former federal judge who was convicted by the Senate of extorting a bribe in a case before him. Four years after Hastings was removed from office as a judge, he was elected to Congress.
4) How long does impeachment take?
In Nixon's case, nine months elapsed between the start of the House Judiciary Committee's impeachment investigation in October 1973 and the committee's approval of its first impeachment resolution. Nixon resigned in early August 1974.
In Clinton's case, the House moved much faster. In September 1998, the House received a report from independent counsel Kenneth Starr that recommended impeachment against Clinton. The House voted to impeach Clinton in December 1998, and the Senate acquitted him in February 1999.
5) What lessons could Democrats draw from the impeachment investigations of Nixon and Clinton?
The Nixon investigation seems to bolster an argument made by Senator Elizabeth Warren, D, that if Democrats think Trump deserves to be impeached, they ought to try.
Public support for Nixon's removal was low at the start of the investigation but rose steadily as the probe uncovered new evidence of his abuses of power. His resignation brought a wave of public revulsion with Washington corruption - and a huge political boost to Democrats. The 1974 elections swept in a wave of "Watergate Baby" legislators who gave Democrats huge advantages in the House and Senate.
Clinton's impeachment, however, did not turn out as well for the opposition party.
In the election held in the middle of their impeachment investigation, Republicans were accused of overreach and lost seats in the House. Speaker Newt Gingrich, R, who had led the charge, resigned after unrest in his caucus.
Neither of these cases, however, is a very useful case study for today's Democrats - since Clinton and Nixon were both in their second terms.
Trump is in his first. That has led some Democrats to conclude that they should focus more on defeating Trump in 2010 than impeaching him before then.
