US Democrats have opened a formal impeachment inquiry into President Donald Trump over allegations he sought help from a foreign power to damage a political rival.
Top Democrat Nancy Pelosi said the president “must be held accountable”.
Mr Trump denies impropriety and called the efforts “witch hunt garbage”.
While there is strong support from Democrats on impeachment, if the inquiry moves forward it is unlikely to pass the Republican-controlled Senate.
The row was sparked by reports an intelligence whistleblower lodged a formal complaint about a phone call President Trump made with Ukrainian counterpart Volodymyr Zelensky.
What exactly was said remains unclear but Democrats accuse Mr Trump of threatening to withhold military aid to force Ukraine to investigate corruption allegations against former Vice-President Joe Biden and his son Hunter.
Mr Trump has acknowledged discussing Joe Biden with Mr Zelensky but said he was only trying to get Europe to step up assistance by threatening to withhold military aid.
Ms Pelosi said Mr Trump had committed “a violation of the law”, and 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.”
As House Speaker Ms Pelosi is the most senior Democrat. She has so far resisted calls among her liberal rank-and-file to attempt to remove the Republican president from office as such an effort could bolster his support.
Mr Biden has denied wrongdoing and also backed 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.
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”.
In his response, House Republican Leader Kevin McCarthy said: “Speaker Pelosi happens to be the Speaker of this House, but she does not speak for America when it comes to this issue.”
“She cannot unilaterally decide we’re in an impeachment inquiry,” he added.
Meanwhile, the acting director of US national intelligence, Joseph Maguire, has refused to share the whistleblower report with Congress. He is due to testify before a public House intelligence committee hearing on Thursday.
According to the New York Times, the whistleblower has expressed an interest in talking to lawmakers and the White House and intelligence officials are working on a deal to allow the unnamed person to speak to Congress.
For months now, Democratic leaders in the House of Representatives have been playing a semantics game. They wanted those who supported and those who opposed a formal impeachment inquiry into President Donald Trump to both think they were getting what they wanted.
This strategy suggested a fear by Speaker Nancy Pelosi and others that heading down the path to impeachment would put moderate Democrats facing tough 2020 re-election fights at risk.
That calculation appears to have changed, after the rapid drumbeat of new revelations about Mr Trump’s contacts with Ukrainian President Volodymyr Zelensky. Now even middle-of-the road politicians are coming out in favour of impeachment proceedings.
The dam has broken. The genie is out of the bottle. Pick your metaphor. The simple fact is that Ms Pelosi – a keen judge of the political mood within her caucus – has made the decision to shift from resisting impeachment to – at the very least – being open to it.
The path forward is uncertain. The president has announced that he will release the transcript of his 25 July phone conversation with Zelensky. While that won’t be enough for Democrats, perhaps the White House will do more to accede to Congress’s requests.
Opinion surveys could show the latest drama is taking a toll on one party or the other, causing political will to crumble. Or, both sides could dig in for a long, gruelling battle that could drag into the darkest days of winter.
More from Anthony on why Ms Pelosi has made her move
Ms Pelosi’s announcement gives an official go-ahead for a committee to investigate the US president’s phone call with the Ukrainian leader and determine whether he committed an impeachable offence.
In her announcement she said the six other congressional committees investigating Mr Trump on other matters would continue under the umbrella of a formal impeachment inquiry.
If it moves forward the House of Representatives will vote on any charges and with the Democrats in the majority there, it could comfortably pass.
But it would next move to the Senate, where a two-thirds majority is required – and where the Republicans hold sway.
A YouGov poll said 55% of Americans would support impeachment if it was confirmed that President Trump suspended military aid to Ukraine in order to push the country’s officials to investigate Joe Biden.