Democrats will open a formal impeachment inquiry into President Donald Trump over claims that he sought political help from Ukraine, US media reports say.
The decision by top Democrat Nancy Pelosi follows growing demands from her party.
Mr Trump has denied impropriety but has acknowledged discussing political rival Joe Biden with the Ukrainian president.
No US president has ever been removed from office by impeachment.
There has been no official confirmation from Ms Pelosi, who as House Speaker is the most senior Democrat. But an announcement is due shortly.
Mr Biden has backed impeachment proceedings unless the US president complies with investigations into his conversation with Ukraine’s leader.
Impeaching Mr Trump “would be a tragedy”, Mr Biden said. “But a tragedy of his making.” The former vice-president is frontrunner to take on Mr Trump in the 2020 election.
There is widespread support among Democrats in the lower house for impeachment with more than 145 out of 235 members in favour.
But if impeachment moves forward it is unlikely to pass the Republican-controlled Senate and opinion polls show it is unpopular among US voters.
What is this row about?
Last week reports said US intelligence officials had complained to a government watchdog about Mr Trump’s interactions with a foreign leader, who was later revealed to be the president of Ukraine, Volodymyr Zelensky.
That whistleblower’s complaint – which was deemed “urgent” and credible by the intelligence inspector general – has been demanded by Democrats in Congress, but the White House and Department of Justice have refused to provide it.
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 Mr 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.
The dam has broken
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 calculus 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.