Why is Donald Trump (potentially) being impeached? Everything you need to know.

The US House of Representatives will launch a formal impeachment inquiry into US 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 has spilled into the 2020 presidential campaign.

House Speaker Nancy Pelosi announced the inquiry on Tuesday after a closed-door meeting with Democratic lawmakers, 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 for months had been reluctant to embrace an impeachment effort.

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

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

Trump promised on Tuesday to release a transcript of his phone call. He has acknowledged he discussed Biden in the call, but denied he withheld nearly $US400 million ($A588 million) in US aid to Ukraine as leverage to get Zelenskiy to launch a probe that would damage Biden, who leads opinion polls in the Democratic race to face Trump in the November 2020 election.

Watch: If Trump worked in your office. Post continues after video.

Pelosi said the six congressional committees currently investigating Trump would continue with their probes as part of the inquiry.

“The actions of the Trump presidency revealed a dishonourable fact of the president’s betrayal of his oath of office, betrayal of our national security and betrayal of the integrity of our elections,” Pelosi said.

The impeachment inquiry could eventually lead to Trump’s removal from office. But even if the Democratic-controlled House voted to impeach Trump, the Republican-controlled Senate would have to take the next step of removing him from office after a trial.

It will be the first impeachment inquiry in Congress since the 1998 probe of President Bill Clinton for perjury and obstruction of justice in relation to his affair with White House intern Monica Lewinsky.

The House voted to impeach Clinton in December 1998, but the Democratic president was acquitted two months later by the Senate and remained in office.

Biden said he would back impeachment if the president did not fully comply with congressional investigations.

“If we allow a president to get away with shredding the Constitution, that will last forever,” Biden told reporters in Wilmington, in his home state of Delaware.

“The House must impeach,” Senator Elizabeth Warren, the first major contender to call for impeachment following former Special Counsel Robert Mueller’s report into Russian interference in the 2016 election, wrote on Twitter on Tuesday.

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.

Democrats are also seeking the original complaint about Trump’s call, filed by a whistleblower within the US intelligence community, as well as information on deliberations over the Ukrainian aid.

The US Senate approved a resolution on Tuesday calling on the Ukraine whistleblower complaint to be submitted to the Senate and House Intelligence committees.

Trump administration officials so far have refused to let the complaint be submitted.