The announcement on Tuesday of a formal impeachment inquiry by the Democrats in the House of Representatives into United States President Donald Trump for abuse of power is a high-stakes gambit with uncertain consequences.
How does impeachment work?
Any member can introduce an impeachment resolution which, like any other bill, is sent to a committee. The process can also be started without a resolution, as with the current impeachment inquiry.
The committee can review the evidence it receives, or carry out an investigation itself.
If the evidence is strong enough, the committee crafts articles of impeachment — the political equivalent of criminal charges — and sends them to the full House.
The House can pass the articles by a simple majority vote, "impeaching" the president.
The articles then go to the Senate, where a trial takes place, with representatives from the House acting as prosecutors and the president and his attorneys presenting his defence.
The chief justice of the Supreme Court presides over the trial in the Senate.
The 100-member Senate then votes on the charges, with a two-thirds majority necessary to convict and remove the president.
If the president is convicted, the vice president then takes over the White House.
Has a US president been impeached before?
No president has been ousted from office by impeachment, but even the threat can bring one down — Richard Nixon resigned in 1974 to avoid certain removal in the Watergate scandal.
Two presidents beat the process: the House formally impeached Andrew Johnson in 1868 and Bill Clinton in 1998, but in both cases they were acquitted in the Senate.
If lawmakers believe a president is guilty of what the US Constitution calls "treason, bribery, or other high crimes and misdemeanours", the process begins in the House of Representatives.
In the cases of Mr Clinton and Nixon, independent prosecutors conducted extensive investigations and amassed evidence to support criminal charges.
Nixon was accused of obstruction of justice, abuse of power, and contempt.
Mr Clinton, in the Monica Lewinsky scandal, was accused of perjury and obstruction.
Is it likely Trump will be impeached?
Mr Trump could conceivably face charges of abuse of power for using his office to pressure Ukraine to conduct a politically-motivated investigation of Joe Biden and his son, Hunter, who had business dealings in Ukraine.
Special Counsel Robert Mueller, in the Russia election meddling investigation, also detailed multiple instances of alleged obstruction of justice by Mr Trump that could arguably support charges.
The charge of high crimes and misdemeanours covers allegations of a range of misconduct — not just ordinary breaches of the criminal code.
Going on holiday for a year, for example, is not illegal but would likely lead to a president's impeachment for failing to discharge his duties under the constitution.
And while strong evidence is required, the impeachment process is very much political in nature, not criminal, and Democrats are divided for political reasons.
Ms Pelosi has argued that impeaching Mr Trump would go nowhere in the Republican-controlled Senate and could damage the party's effort to win full control of the Congress and the White House in the November 2020 elections.
Others in the party say Mr Trump needs to be held accountable — that Democratic voters demand it.