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Media caption“He’s violated so many clauses of the constitution” – US citizens react to impeachment inquiry

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.

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.

More from Anthony on why Ms Pelosi has made her move

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.”

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Media captionPelosi: “The president must be held accountable; no one is above the law.”

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.

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Media captionTrump confirms he withheld aid to Ukraine – but insists there was no “quid pro quo”

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.

What happens next?

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.