House Speaker Nancy Pelosi on Tuesday launched a formal impeachment inquiry into President Donald Trump, a dramatic development triggered by reports that the president pressured Ukraine to investigate a leading 2020 rival and his son.

“The actions of the Trump presidency revealed dishonorable facts 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, noting that the president admitted discussing former Vice President Joe Biden, and his son, Hunter, who had business in Ukraine, during a July telephone call with that nation’s leader.

“The president must be held accountable,” Pelosi said. “No one is above the law.”

Her announcement, aired live from the Capitol with a rank of pressed U.S. flags behind her and a bank of cameras and microphones in front, came toward the end of a whirlwind day that put the White House and Congress on a constitutional collision course.

Even as Trump agreed to release a declassified and unredacted transcript of the phone call at issue, momentum was building for impeachment among the House Democrats who heretofore had remained on the sidelines as their party’s left pushed for confrontation. The speaker herself had long been skeptical of taking such a step.

As Pelosi spoke, Trump was tweeting. “PRESIDENTIAL HARASSMENT!” he wrote. “They never even saw the transcript of the call. A total Witch Hunt!”

Biden, the front-runner for the 2020 Democratic presidential nomination, said Trump should be impeached if he resists congressional probes into whether he withheld military aid for Ukraine as leverage in a bid to force new scrutiny on his son’s business dealings there.

“The president should stop stonewalling this investigation and all the other investigations into his wrongdoing," Joe Biden told reporters at a news conference in Wilmington. “If he continues to obstruct Congress and flout the law, Donald Trump will leave Congress in my view no choice but to initiate impeachment. That would be a tragedy, but a tragedy of his own making.”

Trump has insinuated that while vice president in the Obama administration Biden had threatened to withhold billions from Ukraine to get the nation’s prosecutor general to stop investigating his son, who had been hired as a board member by a Ukranian gas company. That accusation has been debunked by independent fact-checkers.

» READ MORE: Philly-area Democrats mostly line up in support of impeachment inquiry

Tuesday’s events came two days before acting Director of National Intelligence Joseph Maguire was scheduled to brief members of Congress over the heart of the Ukrainian controversy — a whistle-blower complaint from an intelligence community member. Maguire has refused to turn over that complaint.

Trump said on Twitter he had authorized the Wednesday release of the “complete, fully declassified and unredacted” transcript of the July phone call with Ukraine President Volodymyr Zelensky. But Democratic lawmakers have demanded access to the full complaint, not just a call transcript.

Rep. Adam Schiff, a California Democrat and chairman of the House Intelligence Committee, announced Tuesday that the attorney for the whistle-blower has expressed interest in having the intelligence community member testify before Congress.

Pelosi had long resisted pressure from the party’s left wing to impeach Trump in connection with Russian interference in the 2016 campaign and other matters. But on Tuesday afternoon, she telegraphed a change in her position.

"Now that we have the facts, we’re ready,” Pelosi said at a forum hosted by The Atlantic magazine and broadcast on CNN.

» READ MORE: What impeachment is and how the process works

In recent months, the White House has stonewalled other requests from House Democrats for documents on multiple issues.

The Constitution says the president may be removed from office on conviction for treason, bribery, or other “high crimes and misdemeanors." The House decides whether to impeach, which is similar to an indictment, and the Senate hears the evidence and sits as jury; a two-thirds vote is required for conviction.

The House is controlled by Democrats and the Senate by Republicans who have often defended the president, so gridlock is possible on the question of removal — if the House decides to impeach.

Pelosi said she asked the leaders of six committees investigating various accusations of wrongdoing against Trump to compile their arguments for impeachment, which will then be considered by the House Judiciary Committee. Judiciary will then decide whether there should be a floor vote on impeachment. The speaker did not provide a timeline.

Michael Gerhardt, who is the National Constitution Center’s scholar-in-residence and the author of Impeachment: What Everyone Needs to Know, said that it was significant that Pelosi had just openly authorized a formal impeachment investigation, but any potential removal of the president “is pretty far down the road.”

The investigation of President Richard Nixon took two years before it began to yield evidence that Congress deemed worthy of articles of impeachment, said Gerhardt, who is also the inaugural Richard Beeman Visiting Scholar at the University of Pennsylvania Law School.

One important aspect of impeachment that many people misunderstand is that the process does not require an alleged criminal offense, Gerhardt said. Impeachment was envisioned as a way “to get at misconduct that wasn’t indictable,” such as an abuse of power, he said.

Impeachment also is an inherently political act, so public support is important, Gerhardt said. If the House Democratic leadership’s newfound urgency for impeachment “doesn’t diminish, that’s telling you something,” he said.

Trump’s alleged attempt to solicit dirt on a rival via a foreign government has echoes of 2016, when his aides met with Russian officials and Trump openly cheered an illegal hack of Democratic computers, all while using the resulting information to accuse Hillary Clinton and her allies of corruption — much as he has reportedly tried to do this year to Biden.

Biden and his campaign have been seeking the right balance of response to Trump on the Ukraine issue. He criticized Trump as divisive during an appearance at a fund-raiser Monday at the Franklin Institute, where he spoke for about 30 minutes to about 100 donors but did not take questions or mention Ukraine.

Still, his campaign sent supporters a fund-raising email before that appearance, calling Trump’s actions “an unprecedented abuse of power that requires an unprecedented response."

That was followed by a second fund-raising email shortly after the appearance that said, “After abusing the powers of the presidency to ask for dirt from a foreign leader EIGHT times, Trump is not only admitting to his actions, he’s doubling down that he’s RIGHT.”

Trump’s campaign responded in kind with its own fund-raising email to supporters shortly before Biden spoke Tuesday.

"The Democrats know they have no chance of winning in 2020, so now they are crying, ‘Impeachment!’ ” the email said. “There are now over 150 House Democrats who back Impeachment. We CANNOT let these hateful and baseless attacks go on any longer.”

Staff writer Robert Moran contributed to this article.