WASHINGTON (Reuters) - Democrats in the U.S. House of Representatives on Tuesday launched a formal impeachment inquiry into President Donald Trump, accusing him of seeking foreign help to smear Democratic rival Joe Biden ahead of next year’s election.

The inquiry ensures a partisan fight in Congress and on the presidential campaign trail in coming months, energizing the most committed Republican and Democratic supporters and overshadowing the Democratic race for the nomination to face Trump in the November 2020 election.

House Speaker Nancy Pelosi announced the inquiry after a closed-door meeting with Democratic lawmakers, saying Trump’s actions undermined national security and violated the U.S. 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 Biden, the Democratic presidential front-runner, and his son Hunter, who had worked for a company drilling for gas in Ukraine.

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 U.S. 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 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.

“The actions of the Trump presidency revealed a dishonorable 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 Trump accused Pelosi of playing politics with the decision. Republican Senate Majority Leader Mitch McConnell called it “a rush to judgment,” 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 in a statement.

The impeachment inquiry could eventually lead to 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.

U.S. stocks plunged in volatile trade on Tuesday, with the S&P 500 posting its biggest daily drop in a month as investors were rattled by Pelosi’s announcement.

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

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“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 U.S. Senators Elizabeth Warren, Bernie Sanders, Kamala Harris, Cory Booker and Amy Klobuchar; former U.S. Representative Beto O’Rourke; South Bend, Indiana, Mayor Pete Buttigieg; and former U.S. 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.

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

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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 Trump’s call, filed by a whistleblower within the U.S. 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.

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 U.S. aid in exchange for a probe. Quid pro quo is a Latin phrase meaning a favor that is exchanged for a favor.

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,” Trump told reporters at the United Nations General Assembly in New York.

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.”