The push among Democrats in the US Congress to impeach President Donald Trump for abuse of power is gaining momentum.
Nancy Pelosi, the Democratic speaker of the House of Representatives, has shown little appetite for impeachment during the first three years of Trump's tumultuous presidency.
But the political scandal over Trump's attempt to seek dirt from Ukraine on his potential 2020 Democratic presidential rival Joe Biden may be the last straw.
More than 150 of the 235 Democratic members of the 435-seat House support impeachment or the opening of an inquiry into removing the president.
No House Republicans have come out in favor of impeachment and Republicans currently control the Senate, making conviction unlikely.
Trump on Tuesday denounced calls for his impeachment as "ridiculous" and "nonsense."
"It's a witch hunt," he said.
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.
How does it work?
If lawmakers believe a president is guilty of what the US Constitution calls "treason, bribery, or other high crimes and misdemeanors," the process begins in the House of Representatives.
Any member can introduce an impeachment resolution, which like any other bill would be sent to a committee, most likely the House Judiciary Committee.
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 -- 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 defense.
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 would then take over the White House.
What kind of charges do presidents face?
The accusations have to meet the constitutional standard of "high crimes or misdemeanors," which is very broad.
In the cases of 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.
Clinton, in the Monica Lewinsky scandal, was accused of perjury and obstruction.
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 Trump that could arguably support charges.
Is it about law or politics?
Both. Given the momentous nature of an effort to remove the president, a clear crime with strong evidence -- stronger than for an average citizen -- is required.
At the same time, it is very much a political decision.
In past impeachments, support and opposition ran along party lines, though in Nixon's case the offenses were so egregious that Republican backing for him quickly disintegrated.
In Democrat Clinton's case, Republicans controlled the entire Congress. But when impeachment charges went to the Senate, the 45 Democratic Senators stayed united to block a two-thirds vote for conviction.
With Trump, Democrats are divided for political reasons.
Pelosi has argued that impeaching 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 Trump needs to be held accountable -- that Democratic voters demand it.