House Speaker Nancy Pelosi, D-Calif., announced the formalization of an impeachment inquiry into President Trump Tuesday evening, saying "the president must be held accountable" for his "betrayal of his oath of office, betrayal of our national security, and the betrayal of the integrity of our elections."

The speaker effectively endorsed the process, which to some degree has already been underway, after facing fresh pressure from inside the caucus to act. The move could help Democrats' disputed arguments in court that impeachment proceedings were in fact in progress, which could entitle Congress to obtain additional documents.

Pelosi specifically charged that the administration had violated the law by not turning over a whistleblower complaint concerning Trump's July call with Ukrainian President Volodymyr Zelensky. Citing testimony that the director of national intelligence was blocking the release of that complaint, she said: "This is a violation of law. The law is unequivocal."

Trump allegedly pushed Zelensky to investigate former Vice President Joe Biden and his son Hunter. Joe Biden has acknowledged on camera that, when he was vice president, he successfully pressured Ukraine to fire its top prosecutor, Viktor Shokin, while Shokin was investigating the natural gas firm Burisma Holdings — where Hunter Biden was on the board. Shokin himself had separately been accused of corruption.

But there were several indicators throughout the day that Pelosi's gambit could backfire, as Republicans predicted over the weekend.

"It is a colossal error." — Texas Republican Sen. John Cornyn

Trump vowed earlier Tuesday to release a "complete" transcript of his call with Zelensky by Wednesday.

Additionally, a senior administration official told Fox News late Tuesday that the White House is working as quickly as it can to release to Congress a whistleblower complaint, as long as it's legally possible. The official told Fox News that the White House had nothing to hide, that there has been no wrongdoing,

The Wall Street Journal reported that the complaint must be declassified first, but will likely be in Congress' hands by the end of the week.

And The New York Times reported that the White House had dropped its objection to the whistleblower speaking to Congress. That came after the GOP-controlled Senate passed a unanimous resolution seeking access to the whistleblower's complaint.

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Still, Pelosi appeared undeterred -- and she and other Democrats suggested to reporters that impeachment could be appropriate even for matters unrelated to the Ukraine probe.

"This week, the president has admitted to asking the president of Ukraine to take actions which would benefit him politically," Pelosi said. "Therefore, today, I'm announcing the House of Representatives is moving forward with an official impeachment inquiry. I'm directing our six committees to proceed with their investigations under that umbrella.

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

Invoking the "darkest days of the American Revolution," Pelosi called on lawmakers to honor their constitutional oath to protect the country "from all enemies, foreign and domestic."

The Democratic-led committees then would gather evidence and present it to Pelosi and House Judiciary Committee Chairman Jerry Nadler, D-N.Y., who would make the ultimate determination on whether to hold an impeachment vote.

A Democratic source told Fox News that the House Intelligence Committee, led by California Democratic Rep. Adam Schiff, would take the lead in the investigation -- and that Nadler might not be pleased with the arrangement.

“I don’t think he’s happy that he has less influence here, but everybody is on the same page and has the same role,” the source said of Nadler.

Schiff “will have a heavy role here because the foundation of the Ukraine allegations is thorough his committee," the source said.

House Minority Leader Kevin McCarthy, R-Calif., responded to Pelosi's statement by noting that Democrats have been pushing to impeach Trump for years, for a variety of reasons.

"Nancy Pelosi may be Speaker of the House," McCarthy said, "but she does not speak for America. ... What she said today made no difference in what's been going on. This election is over. It's time to put the public before politics."

Trump, for his part, ripped into Democrats in a series of tweets immediately after Pelosi's comments, writing that "PRESIDENTIAL HARASSMENT" was in progress again.

"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," Trump wrote. "So bad for our Country!"

He added: "They never even saw the transcript of the call. A total Witch Hunt!"

Trump is set to meet with Zelensky in New York on Wednesday. The visit was previously scheduled, unrelated to the whistleblower allegation, although the two leaders are expected to face questions about the matter from reporters.

House Oversight Committee Ranking Member Jim Jordan, R-Ohio, called Democrats' efforts predictable and destructive in his own fiery statement.

"Democrats have been trying to impeach the President since the beginning of this Congress," Jordan said. "Michael Cohen's testimony was a bust. John Dean's testimony was a waste of time. The Mueller report did not live up to the hype.

"Speaker Pelosi's decision to pursue impeachment now - on the basis of unsubstantiated, indirect, and anonymous allegations - only shows that the Speaker has finally succumbed to unrelenting pressure from the socialist wing of the Democrat Party," Jordan added. "This was never about Russian collusion or Ukrainian prosecutions. It is all about undoing the 2016 election and the will of the American people."

Pelosi had met Tuesday afternoon with her caucus of House Democrats prior to giving the planned statement.

Wary of compromising the electability of vulnerable Democrats in the caucus, Pelosi had long resisted taking the dramatic step publicly -- even though other top Democrats have insisted, in legal filings and the media, that an impeachment inquiry already has been in progress for months.

Swing district Rep. Mikie Sherrill, D-N.J., acknowledged to Fox News that supporting the impeachment inquiry "could" affect her electorally, but she maintained that Trump voters in her district "understand," and that Trump crossed a red line.

Republicans said the move would prove to be a major political mistake.

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"It is a colossal error," Texas Republican Sen. John Cornyn told Fox News just prior to Pelosi's comments. "And, I’m kind of surprised that Speaker Pelosi, as shrewd as she is, would let it get to this point."

Pelosi's impeachment imprimatur came after a flurry of those swing-district Democrats late Monday wrote an op-ed in The Washington Post backing an impeachment inquiry, or a fact-finding look at whether an impeachment vote by the full House is warranted. And on Saturday night, Rep. Alexandria Ocasio-Cortez, D-N.Y., said Democrats' failure to act on impeachment had been a "national scandal."

Pelosi's decision Tuesday heartened Democrats who have long called for impeaching the president, only to be stalled after Special Counsel Robert Mueller found no evidence of Russian collusion by the Trump campaign.

But, new momentum built quickly for the move after news of the whistleblower complaint surfaced.

In remarks to reporters at the United Nations on Monday, Trump denied linking the aid money to Ukraine's investigative actions. “No, I didn’t — I didn’t do it,” Trump said. But, he also repeatedly called the Bidens' actions in Ukraine a "disgrace," acknowledged that Biden had come up during the call and added: "It's very important to talk about corruption. ... Why would you give money to a country that you think is corrupt?"

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Trump tweeted Tuesday that he would make a "complete" transcript of his phone call public on Wednesday.

"I am currently at the United Nations representing our Country, but have authorized the release tomorrow of the complete, fully declassified and unredacted transcript of my phone conversation with President Zelensky of Ukraine," Trump wrote. "You will see it was a very friendly and totally appropriate call. No pressure and, unlike Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!"

Justice Department lawyers, as well as lawyers at the White House, have been advising White House officials to release the transcript since last week, a source familiar with the conversations told Fox News.

At an event Tuesday, Pelosi intimated that impeachment would remain on the table, regardless of what the transcript showed. Many conservatives charged that she was moving the goalposts and lowering expectations.

"We have many other, shall we say, candidates for impeachable offenses in terms of the Constitution, but this one is the most understandable by the public," Pelosi said, referring to the Ukraine phone call allegation. "It's really important to know this: There is no requirement that there be a quid-pro-quo in the conversation."

Other prominent Democrats also seemingly said Trump should be impeached no matter what.

"The president has committed several impeachable offenses," Ocasio-Cortez told reporters after Pelosi's remarks on impeachment. In another indication that Democrats were apparently hedging their bets on the Ukraine matter, Ocasio-Cortez said alleged Emoluments Clause violations by the president could be included in prospective articles of impeachment.

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Democrats continued to push for the whistleblower to testify and to see the full whistleblower complaint throughout the day. Schiff, D-Calif., said Tuesday afternoon that testimony from the whistleblower might be imminent.

"We have been informed by the whistleblower’s counsel that their client would like to speak to our committee and has requested guidance from the Acting DNI [Director of National Intelligence] as to how to do so," Schiff said in a tweet. "We‘re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week."

It was unclear if Acting DNI Joseph Maguire would provide that guidance. Maguire is expected to testify himself before the House Intelligence Committee at an open hearing on Thursday -- but Maguire has refused to share some information, including the complaint.

A source familiar with the matter told Fox News this week that the whistleblower had no firsthand knowledge of Trump's call with Zelensky.

Late Monday, attorneys for the whistleblower released a statement confirming Schiff's statement, and noting that they "await a timely response from the Acting Director providing such guidance."

"We support the bi-partisan, unanimous resolution passed by the Senate regarding our client’s lawful whistleblower complaint and call upon the Acting Director of National Intelligence to transmit the complete disclosure to the two Intelligence Oversight Committees," the lawyers noted.

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As Pelosi met with her caucus, Senate Minority Leader Chuck Schumer, D-N.Y, called for the Senate and House Intelligence Committees to immediately have access to the whistleblower complaint. The panels "should be allowed to evaluate the complaint in a deliberate and bipartisan manner consistent with applicable statutes and processes in order to safeguard classified and sensitive information," Schumer asserted.

Pelosi and House Majority Leader Steny H. Hoyer, D-Md., also released a joint statement Tuesday announcing an imminent resolution condemning the White House's failure to turn over all related documents to Congress.

"Allegations that the President of the United States sought to enlist a foreign government to interfere in our democratic process by investigating one of his political rivals – and may have used the withholding of Congressionally-appropriated foreign assistance days earlier as intimidation – are deeply alarming," Hoyer and Pelosi wrote.

They demanded the whistleblower's protection, and for "the Acting Director of National Intelligence [to] provide Congress the [whistleblower] complaint, as specified under the law, and all requests for documents and testimony relating to this allegation."

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Pelosi and Hoyer continued: "On Wednesday, the House will vote on a resolution making it clear Congress’s disapproval of the Administration’s effort to block the release of the complaint and the need to protect the whistleblower. This is not a partisan matter, it’s about the integrity."

Meanwhile, Yahoo News reported that the GOP-controlled Senate Intelligence Committee had written to the whistleblower's attorney, requesting a closed-door interview.

The precise implications of Pelosi's shift on impeachment were unclear, given Democrats' previous rhetoric on the topic. Earlier this month, Hoyer contradicted some of his colleagues by insisting that Congress is not engaged in any kind of "impeachment inquiry" -- before reversing course hours later in a "clarifying" press statement.

Other top Democrats, including Nadler, have insisted repeatedly in court and in public that an impeachment inquiry has been in progress.

“It has been an impeachment inquiry,” Nadler has said. “What we are doing is clear. It has been very clear. It continues to be very clear."

But, when asked if he thought it was an impeachment inquiry, Hoyer initially responded with an emphatic “No.”

Hoyer added: “I think the delineation ought to be whether or not they are considering a resolution of impeachment.”

The Justice Department, in court filings earlier this month, sought to block congressional Democrats' bid for secret grand jury material from the Mueller investigation by citing the confusion inside the caucus over whether or not they're pursuing an "impeachment investigation."

"The committee’s own description of its investigation makes clear that it is too far removed from any potential judicial proceeding to qualify," the DOJ said.

A total of 172 House Democrats have now signaled strong support for an impeachment inquiry -- 235 Democrats and 198 Republicans are in the House, with one pro-impeachment independent. A majority would be required to successfully impeach the president. A highly unlikely two-thirds vote in the GOP-controlled Senate would be needed to convict and remove the president.

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Vice President Mike Pence would then take office in that scenario.

"The ironic thing is is that everything that our critics in the media are leveling at the president from this phone call, and leveling at our administration, everything that Democrats on Capitol Hill are running off and describing -- Vice President Joe Biden bragged about -- which was a quid-pro-quo -- withholding American aid in exchange for a specific action," Pence told Fox News' "Hannity" on Monday.

Fox News' Chad Pergram, Brooke Singman, Ronn Blitzer and Jake Gibson contributed to this report.