WASHINGTON — House Speaker Nancy Pelosi, who for months resisted efforts to launch impeachment proceedings against President Donald Trump, announced a formal inquiry on Tuesday, saying that the president’s growing Ukraine scandal marked a “breach of his Constitutional responsibilities."

"This week the president has admitted to asking the president of Ukraine to take actions which would benefit him politically," Pelosi said.

"The actions of the Trump presidency revealed the 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," she continued. "Therefore, today I am announcing the House of Representatives is moving forward with an official impeachment inquiry."

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Pelosi said she was formally directing the House's six committees that have jurisdiction over impeachment, oversight and other related matters to "proceed with their investigations under that umbrella."

"The president must be held accountable," she said. "No one is above the law."

Pelosi's change of heart comes as dozens of House Democrats — now more than two-thirds of the caucus — have come out in support of an impeachment inquiry in the wake of reports that Trump may have withheld aid to Ukraine to pressure officials there to investigate the son of political rival Joe Biden.

The impeachment drive follows days of revelations surrounding Trump's apparent push to have the Ukrainian government investigate the former vice president's son Hunter Biden, who had business dealings in the country. On Monday, The Washington Post and other media outlets reported that Trump instructed his acting chief of staff to place a hold on about $400 million in military aid for Ukraine in the days before a late July phone call with Ukrainian President Volodymyr Zelensky.

Trump responded on Twitter within moments of Pelosi’s announcement, calling it “Witch Hunt garbage.”

“They never even saw the transcript of the call. A total Witch Hunt!” he wrote.

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, Nadler, Schiff and, of course, Maxine Waters! Can you believe this? — Donald J. Trump (@realDonaldTrump) September 24, 2019

They never even saw the transcript of the call. A total Witch Hunt! — Donald J. Trump (@realDonaldTrump) September 24, 2019

Earlier, Trump told reporters that an impeachment inquiry would help him in the 2020 election but would harm the country.

“If she does that they all say that’s a positive for me in the election. You could also say who needs it, it’s bad for the country," he said.

Top Republicans expressed frustration at Pelosi's move.

“Washington Democrats have been searching for ways to reverse their 2016 election defeat since before President Trump was even inaugurated," Senate Majority Leader Mitch McConnell, R-Ky., said in a statement.

“The result has been a two-and-a-half-year impeachment parade in search of a rationale. When investigations by Special Counsel Mueller and the Senate Select Committee on Intelligence deflated their breathless accusations of a campaign conspiracy with Russia, Democrats have simply shifted to new arguments for their predetermined conclusion," he added.

Rep. Doug Collins, R-Ga., the top Republican on the House Judiciary Committee, said on Twitter that Pelosi’s announcement “changes absolutely nothing.”

“Merely claiming the House is conducting an impeachment inquiry doesn’t make it so. Until the full House votes to authorize an inquiry, nobody is conducting a formal inquiry,” he said.

Later, in brief remarks to reporters, House Minority Leader Kevin McCarthy, R-Calif., criticized Democrats for “investigating this president even before he got elected.”

“I realize 2016 did not turn out the way Speaker Pelosi wanted,” he said.

As of Tuesday afternoon, more than 175 Democrats supported some type of impeachment action — more than three-quarters of the 235-member caucus. An op-ed article by seven freshman House Democrats in The Washington Post calling for impeachment hearings to address allegations about Trump and Ukraine had been expected to give Pelosi the "cover" she needed to back a more formal impeachment proceeding against the president, three sources familiar with the matter told NBC News on Monday night.

Senate Minority Leader Chuck Schumer, D-N.Y., told reporters Tuesday afternoon that he felt Pelosi was handling the situation "appropriately."

The late July discussion between Trump and Zelensky has been scrutinized following a whistleblower complaint by a member of the U.S. intelligence community that media reports said was tied to the call between the two leaders. The White House is preparing to turn over the whistleblower complaint by this Thursday, according to a senior administration official.

That official tells NBC News the complaint will undergo a classification review and be provided to Congress by Thursday.

Over the weekend, Trump admitted he discussed Biden, a possible 2020 challenger, with Zelensky. On Tuesday, he said he would release the transcript of the call and confirmed reports that his administration temporarily froze almost $400 million in aid to Ukraine. But he gave a new reason for doing so: He said he wanted European countries to contribute money, too, and did not want the United States to do so alone.

Pelosi, for her part, said Tuesday that Trump's alleged intervention marked actions that "undermine both our national security and our intelligence, our protections of whistleblowers."

Trump on Tuesday denied putting any pressure on the Ukrainian leader to probe a political rival.

"I put no pressure on them whatsoever," he said. "I could have. I think it would probably, possibly have been OK if I did. But I didn't. I didn't put any pressure on them whatsoever."

Trump's personal attorney Rudy Giuliani, however, has engaged in a months-long effort to have Ukraine probe the Bidens — an effort aided by the State Department.

That effort centers on Biden's calls, widely backed by the international community, for Ukraine to crack down on corruption during his time as vice president. That included a call to remove a Ukrainian prosecutor, Viktor Shokin, who was seen as ineffective and was later removed by the country's Parliament. One of the cases that Shokin was investigating involved Burisma Holdings, a natural gas company, whose board at the time included Biden's son.

Earlier this year, Bloomberg News, citing documents and an interview with a former Ukrainian official, reported the Burisma investigation had been dormant for more than a year by the time Biden called for the crackdown on corruption. The then-Ukrainian prosecutor general told the news agency he found no evidence of wrongdoing by Biden and his son. And PolitiFact reported it found no evidence to "support the idea that Joe Biden advocated with his son's interests in mind."

Heidi Przybyla reported from Washington, and Adam Edelman from New York.