United States of America’s House of Representatives Speaker Nancy Pelosi on Tuesday, Sep 24, announced a formal impeachment inquiry into Donald Trump over allegations of seeking foreign help to smear Democratic rival Joe Biden ahead of next year’s election.

The move came after mounting pressure from fellow Democrats, plunging a deeply divided nation into an election year clash between Congress and the President.

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

In a deeply divided country, the inquiry means there would be highly partisan fight in Congress and a polarised presidential campaign in coming months.

House Speaker Nancy Pelosi announced the inquiry after a closed-door meeting with Democratic lawmakers, saying Donald Trump’s actions undermined national security and violated the US Constitution.

Trump, who was meeting with world leaders at the United Nations, previewed his defense in an all-caps tweet: “PRESIDENTIAL HARRASSMENT!”

He called the inquiry “Witch Hunt garbage”.

Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our Country!

— Donald J. Trump (@realDonaldTrump) September 24, 2019

Pelosi’s change of heart followed reports that Donald Trump had pressured Ukrainian President Volodymyr Zelenskiy in a July 25 phone call to investigate Biden, the Democratic presidential front-runner, and his son Hunter, who had worked for a company drilling for gas in Ukraine.

In the days before the call, Trump ordered advisers to freeze $400 million in military aid for Ukraine — prompting speculation that he was holding out the money as leverage for information on the Bidens. Trump has denied that charge, but acknowledged he blocked the funds.

Donald 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 $400 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 presidential race.

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

The panels would work collaboratively, and then decide whether the House Judiciary Committee should draft articles of impeachment, according to media reports quoting House aides.

“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.

Congressional allies of Donald Trump accused Pelosi of playing politics with the decision.

Republican Senate Majority Leader Mitch McConnell called it “a rush to judgement”, saying it should have waited until after details of the phone call were revealed. “It simply confirms that House Democrats’ priority is not making life better for the American people, but their nearly three-year-old fixation on impeachment,” he said.

The impeachment inquiry could eventually lead to Donald Trump’s removal from office, although that would be a steep task for Democrats. Even if the Democratic-controlled House voted to impeach Trump, the Republican-majority Senate would have to take the next step of removing him from office after a trial. A conviction would require a two-thirds Senate majority.

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.

The only other president to be impeached, Andrew Johnson in 1868, was also acquitted by the Senate. President Richard M Nixon resigned in 1974 after the House initiated impeachment proceedings but before an official vote, stepping down rather than face the embarrassment of such a rebuke.

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.

Most Democratic presidential contenders support an impeachment inquiry, including US Senators Elizabeth Warren, Bernie Sanders, Kamala Harris, Cory Booker and Amy Klobuchar; former US Representative Beto O’Rourke; South Bend, Indiana, Mayor Pete Buttigieg; and former US Secretary of Housing and Urban Development Julian Castro.

But the move carries a risk of backfiring on Democrats if voters believe they are overreaching, as it did on House Republicans who unexpectedly lost seats in the 1998 elections after pursuing impeachment against Clinton.

Donald Trump has proven particularly resilient since taking office in January 2017, withstanding repeated scandals and retaining strong support from Republicans.

He promised to release a “complete, fully declassified and unredacted” transcript of the July 25 Ukraine call on Wednesday.

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

House Intelligence Committee Chairman Adam Schiff said his panel was communicating with an attorney representing the whistleblower and that the individual would like to testify this week.

The White House had refused to hand over the whistleblower’s complaint to Congress. But an administration official said late on Tuesday it was now preparing to release it by the end of the week and most likely allow the whistleblower to meet with congressional investigators.

The Senate approved a resolution on Tuesday calling for the complaint to be submitted to the Senate and House Intelligence committees. The House will vote on a similar resolution on Wednesday.

Donald Trump said the transcript would show the Ukraine 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.

Donald Trump has produced no evidence of wrongdoing by Biden or his son.

“When you see the readout of the call, which I assume you’ll see at some point, you’ll understand. That call was perfect. It couldn’t have been nicer,” Donald Trump told reporters at the United Nations General Assembly in New York.

Donald Trump indicated there was nothing sinister about withholding the aid to Ukraine but he wanted Europe and not just the United States to step up and provide Ukraine assistance.

The money was later released by the Trump administration. Trump told reporters he had wanted the money for Ukraine frozen, but changed his mind after “people called me.”