US Democrats have opened a formal impeachment inquiry into President Donald Trump over claims that he sought political help from Ukraine.
The decision by top Democrat Nancy Pelosi follows growing demands from her party. She said the president “must be held accountable”.
Mr Trump has denied impropriety and argued that impeachment would be politically “positive” for him.
No US president has ever been removed from office by impeachment.
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There is widespread support among Democrats in the lower house for impeachment – more than 145 out of 235 members are 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, deemed “urgent” and credible by the intelligence inspector general, has been demanded by Democrats 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.
What did Ms Pelosi say?
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.”
Mr Biden has 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.” The former vice-president is frontrunner to take on Mr Trump in the 2020 election.
How has Mr Trump responded?
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.
Earlier in the day, Mr Trump said “they’re going to lose the election and they figured this is a thing to.”
“If she does that, they all say that’s a positive for me from the election,” he said. “You could also say, who needs it? It’s bad for the country.”
He has promised to release a transcript of his conversation with Ukraine’s president to show it was “totally appropriate”.
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 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.