The US Democratic Party has begun a formal impeachment inquiry into President Donald Trump over allegations that he pressured a foreign power to damage a political rival.
Top Democrat Nancy Pelosi said the president “must be held accountable”.
Mr Trump has denied wrongdoing and called the efforts a “witch hunt”.
There is strong support from House Democrats for impeachment, but the proceedings would be unlikely to pass the Republican-controlled Senate.
If the inquiry moves forward the House of Representatives will vote on any charges and, with the Democrats in the majority, the vote would likely be carried – making Mr Trump the third president in US history to have been impeached.
But the proceedings would likely then stall in the Senate, where the president’s Republican Party holds enough of a majority to prevent him from being removed from office.
Senior democrats, including Ms Pelosi, had previously resisted growing calls from within the party to begin impeachment proceedings, but the party’s leadership united on the issue following the Ukraine controversy.
A whistleblower from within the US intelligence community lodged a formal complaint about one or more phone calls between President Trump and the Ukrainian president Volodymyr Zelensky.
The White House has so far refused calls to make transcripts of the calls public, but Democrats say Mr Trump threatened to withhold military aid to Ukraine unless Mr Zelensky agreed to investigate unsubstantiated corruption allegations against Mr Trump’s leading political rival Joe Biden and his son Hunter.
Mr Trump has admitted discussing Joe Biden with Mr Zelensky but denied that he exerted pressure on the Ukrainian president to investigate his political rival.
Two presidents have been impeached in US history – Andrew Johnson in 1868, and Bill Clinton in 1998. But neither were removed from office by a Senate trial.
Richard Nixon resigned in August 1974 before he could be impeached.
What did Nancy Pelosi say?
House Democratic leader Nancy Pelosi said Mr Trump had committed “a violation of the law”, and she called his actions “a breach of his constitutional responsibilities”.
“This week the president has admitted to asking the president of Ukraine to take action that would benefit him politically,” she said, adding: “The president must be held accountable.”
Ms Pelosi is the most senior Democrat. She had for a long time resisted calls among the party’s progressive rank-and-file to launch impeachment proceedings, but the allegations against Mr Trump concerning Ukraine pushed a number of previously reticent Democrats towards supporting impeachment.
Mr Biden has denied wrongdoing and no evidence has emerged to back up the claims against him. He has also said he supports impeachment proceedings unless the US president complies with investigations.
Impeaching Mr Trump “would be a tragedy”, Mr Biden said. “But a tragedy of his making.” He is the current frontrunner to take on Mr Trump in the 2020 election.
How did Mr Trump respond?
In a series of tweets Mr Trump said Democrats “purposely had to ruin and demean” his trip to the UN “with more breaking news Witch Hunt garbage”.
“They never even saw the transcript of the call. A total Witch Hunt!” he added.
He promised to release a transcript of his conversation with Ukraine’s president on Wednesday to show it was “totally appropriate”.