The United States House of Representatives will launch a formal impeachment inquiry into President Donald Trump over reports he sought foreign help to smear a political rival, setting up a dramatic clash between Congress and the White House that could spill into the 2020 presidential campaign.

House Speaker Nancy Pelosi announced the inquiry on Tuesday after a closed-door meeting with Democratic politicians, saying Trump’s actions appeared to have undermined national security and violated the US Constitution.

“The president must be held accountable. No one is above the law,” said Pelosi, who had for months been reluctant to embrace an impeachment effort.

Trump fired back quickly on Twitter, calling the inquiry “Witch Hunt garbage”.

House Speaker Nancy Pelosi announces the House of Representatives will launch a formal inquiry into the impeachment of US President Donald Trump [Kevin Lamarque/Reuters]

After more than two and one-half years of sharp Democratic criticism of Trump, the formal impeachment quest sets up the party’s most urgent and consequential confrontation with a president who thrives on combat – and injects deep uncertainty in the 2020 White House race. Trump has all but dared Democrats to take this step, confident that the spectre of impeachment led by the opposition party would bolster his political support.

Pelosi’s change of heart followed reports that Trump had pressured Ukrainian President Volodymyr Zelensky in a July 25 phone call to investigate Democratic presidential frontrunner Joe Biden and his son.

Trump promised on Tuesday to release a transcript of his phone call. He also confirmed he had withheld nearly $400m in US aid to Ukraine but denied he did so as leverage to get Zelensky to initiate an investigation that would damage Biden.

‘Full steam ahead’

Pelosi said the six congressional committees currently investigating Trump would continue with their probes as part of the inquiry. Those panels would work collaboratively, and then decide whether the House Judiciary Committee should draft articles of impeachment, House aides said.

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“The urgency of the moment is just full steam ahead and that is different. You will see everything moving ahead very quickly,” Representative Pramila Jayapal, who serves on the House Judiciary Committee, told reporters.

Jayapal said she expects the committee to move now towards drafting articles of impeachment.

“That is clearly the direction we are going,” she said.

Representative Steve Cohen said, “Trump doesn’t even know right from wrong – what he did in Ukraine, talking to the president there about Joe Biden’s son, was to buy a result, clear and simple.”

It remains unclear if the House will vote to authorise their official inquiry as has been done in the past.

Biden on Tuesday called on Trump to fully comply with congressional investigations into the matter or risk impeachment.

“If he continues to obstruct Congress and flout the law, Donald Trump will leave Congress in my view with no choice but to initiate impeachment proceedings,” Biden told reporters in Wilmington, in his home state of Delaware.

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Trump, who has withstood repeated scandals since taking office in January 2017, said a “complete, fully declassified and unredacted” transcript of the July 25 call would be released on Wednesday.

The controversy came to light after a whistle-blower from within the US intelligence community lodged a complaint with an internal watchdog about Trump’s conversation with Zelensky.

House Intelligence Committee Chairman Adam Schiff said his panel was communicating with a lawyer representing the whistle-blower and that the individual would like to testify this week.

Trump said the transcript would show the call was “totally appropriate”, that he had not pressured Zelenskiy to investigate Biden and that there had been no “quid pro quo” for US aid in exchange for a probe. Quid pro quo is a Latin phrase meaning a favour that is exchanged for a favour.

With additional reporting by William Roberts in Washington, DC