The announcement marks the most direct step taken by the House Democratic leader to embrace impeachment proceedings and is a significant escalation in the fight between House Democrats and the President.

“Today, I am announcing the House of Representatives moving forward with an official impeachment inquiry,” Pelosi said in a brief speech in the Capitol, adding, “The President must be held accountable. No one is above the law.”

“Actions taken to date by the President have seriously violated the Constitution,” the House speaker said.

The House speaker, who has long pushed to keep her caucus away from the politically divisive issue, is responding to the seismic shift among Democratic members, following Trump's admission of discussing Vice President Joe Biden and his son in his phone call with the Ukrainian President. Dozens of House Democrats — many from moderate or Trump-won districts — have announced their support for an impeachment inquiry over the past 48 hours.

“This week, the President has admitted to asking the President of Ukraine to take actions which would benefit him politically. The actions of the Trump presidency revealed the 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 in her public remarks.

In advance of that statement, Pelosi and Majority Leader Steny Hoyer also announced plans to vote on a resolution of disapproval on Wednesday for 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.” Their statement did not mention impeachment.

‘Full speed ahead’

Pelosi consulted Tuesday afternoon with the six House Democratic leaders to discuss their presentation to the caucus later in the day, Democratic sources familiar with the issue said. In that closed-door meeting before her public announcement, Pelosi said the six chairmen will continue to investigate under a powerful new umbrella of an impeachment inquiry.

A separate source in the room said the speaker added, “Here we are. A moment of truth. Truth is what this has been about all along.”

“He's taken it to another level of betrayal, therefore, we're moving forward with another level of inquiry,” Pelosi said.

Behind the scenes, Pelosi has encouraged members of her caucus to state their impeachment position now in order to show there is a groundswell in the caucus. She is also conveying that message to freshmen.

On Tuesday, Pelosi also told the caucus that the impeachment inquiry will be done “expeditiously,” but wasn't specific in her timeframe, according to a source in the room.

Rep. Jackie Speier told CNN that the President's actions in regards to Ukraine were so egregious, that there has been a “dramatic” shift by Pelosi.

“I think she's very laser-focused on this offence as being so egregious that it warrants a full-blown impeachment inquiry,” she said.

Asked what the timeline is for this, she said, “swiftly.”

House Judiciary Committee Chairman Jerry Nadler said on Tuesday after Pelosi's remarks, “I only have one thing to say, full speed ahead.”

Tide changing for Pelosi allies and vulnerable House freshmen

Civil rights icon Rep. John Lewis on Tuesday announced his support for impeachment proceedings, arguing that “now is the time to act” and any delay “would betray the foundation of our democracy.”

“We will never find the truth unless we use the power given to the House of Representatives and the House alone to begin an official investigation as dictated by the Constitution,” the Georgia Democrat said in a speech on the House floor. “The future of our democracy is at stake.”

Some of Pelosi's closest allies, including House Reps. Rosa DeLauro of Connecticut and Debbie Dingell of Michigan, have signalled their support for impeachment proceedings — a significant indicator that the speaker could be moving closer to backing the divisive political procedure.

Another crucial development has been multiple high profile freshmen Democrats in districts Trump won in 2016 either coming out publicly to fully endorse the House Judiciary Committee's impeachment inquiry or at least opening the door to proceedings, citing Trump's actions as forcing them to act.

One of those freshman Democrats, Rep. Mikie Sherrill of New Jersey, stressed on CNN “New Day” Tuesday that “none of us ran on impeaching the President.” But she argued that the allegation that Trump might have tried leveraging foreign military aid to advance his reelection efforts was “just a line that I think was too much for all of us.”

Pelosi has cited the House members in GOP-leaning districts as part of her hesitance to embrace impeachment, and the groundswell of statements over the past 48 hours could signal a tipping point in the political calculus.

“If investigations confirm recent reports, these actions represent impeachable offences that threaten to undermine the integrity of our elections and jeopardize the balance of power within the federal government,” Rep. Haley Stevens of Michigan said in a statement Tuesday. Stevens had told CNN last month that she had “mixed reviews from the people of my district” on the issue of whether to move forward with impeachment proceedings.

So far, as many as 165 House Democrats have publicly stated their support for an impeachment enquiry into Trump, according to a CNN count, and that doesn't include the House Democrats who've said in recent days that the President's behaviour is a turning point in Democrats’ oversight into the administration.

Pelosi has, for months, resisted pursuing impeachment because she has sought to protect vulnerable Democrats in GOP-leaning districts who could face voter backlash.

Rep. Jamie Raskin of Maryland, a member of House Judiciary, told CNN on Tuesday morning that he believes articles of impeachment are “inevitable” and said there's a “growing clamour throughout the Democratic caucus” to deal with Trump's “reckless and lawless” behaviour.

Some holdouts remain

But some vulnerable Democrats in 2020 are still treading cautiously on the issue.

Rep. Ben McAdams of Utah told CNN on Tuesday that he's “considering” impeachment, but didn't want to discuss the issue further.

“Before making any judgments, I want to know the facts of what occurred between the President and Ukraine,” McAdams said. “I share Sen. (Mitt) Romney's view that If the President used his position to pressure a foreign power to dig up dirt on a rival for his own personal gain, it would be deeply troubling. I believe it would be a betrayal of the loyalty owed to our country and the Constitution.”

Rep. Max Rose, another endangered House Democrat, told CNN that he has not moved off his opposition to impeachment, but argued that “we have to get to the facts” of the allegations.

“It is impossible to not take these allegations seriously. It is a national security matter,” the New York Democrat said Tuesday on Capitol Hill, adding, “It's incredibly serious … what I'm not going to do is jump to make another declaration.”

Asked if that meant he is opposed to moving ahead on impeachment, Rose said, “My current position right now is clear, but that is not a justification to not take these points as seriously as possible.”

Rep. Susan Wild stood up during the Democratic caucus meeting on Tuesday and argued that Pelosi should cancel some of the upcoming two-week recess to not lose momentum, according to a source in the room.

The House, at the end of the week, will recess for two weeks for a district work period over the Yom Kippur and Columbus Day holidays.

CNN previously reported that Trump pressed Ukrainian President Volodymyr Zelensky in a July 25 call to investigate former Vice President Joe Biden's son, Hunter, according to a person familiar with the situation.

There has been no evidence of wrongdoing by either Joe or Hunter Biden.

That call was also part of a whistleblower complaint submitted to the Intelligence Community Inspector General, which hasn't been shared with lawmakers despite calls from Democrats.

Trump acknowledged on Sunday that he discussed the former Vice President Joe Biden in a phone call over the summer with Ukraine's president, but denied doing anything improper.

CNN confirmed that Trump asked his acting chief of staff, Mick Mulvaney, to put a hold on millions in military aid to Ukraine roughly one week before a call with Zelensky, two senior administration officials said on Monday. The Washington Post first reported this.

In her public remarks on Tuesday, Pelosi said, “The law is unequivocal: it says the DNI, director of national intelligence, shall provide congress the full whistleblower complaint,” Pelosi said.

Earlier this month, the House Judiciary Committee approved a resolution defining the rules of the panel's impeachment investigation. That vote came, however, as Democrats struggled to publicly define the committee probe with Nadler saying the committee is conducting an impeachment inquiry, while Pelosi and other Democratic leaders had refrained from calling it that.

In a nod to the ongoing investigations in the House, Pelosi said on Tuesday, “For the past several months we have been investigating in our committees and litigating in the courts so that the House can gather all the relevant facts and consider whether to exercise its full Article I powers, including a constitutional power of the utmost gravity, approval of articles of impeachment.”

The House Speaker also said in her speech, “I can say with authority the Trump administration's actions undermine both our national security and our intelligence and our protections of whistleblowers.”