WASHINGTON -- U.S. House Speaker Nancy Pelosi has launched a formal impeachment inquiry against President Donald Trump, yielding to mounting pressure from fellow Democrats and plunging a deeply divided nation into an election year clash between Congress and the commander in chief.

The probe focuses partly on whether Trump abused his presidential powers and sought help from a foreign government to undermine Democratic foe Joe Biden and help his own reelection effort. Pelosi said Tuesday such actions would mark a "betrayal of his oath of office" and declared, "No one is above the law."

The impeachment inquiry, after months of investigations by House Democrats of the Trump administration, sets up the party's most direct and consequential confrontation with the Republican president, injects deep uncertainty into the 2020 election campaign and tests anew the nation's constitutional system of checks and balances.

Trump, who thrives on combat, has all but dared Democrats to take this step, confident that the spectre of impeachment led by the opposition party will bolster rather than diminish his political support.

"There has been no President in the history of our Country who has been treated so badly as I have," he tweeted Wednesday from New York, where he has spent the week meeting with world leaders participating in the annual UN General Assembly. "The Democrats are frozen with hatred and fear. They get nothing done. This should never be allowed to happen to another President. Witch Hunt!

While Pelosi's announcement adds weight to the work being done on the oversight committees, the next steps are likely to resemble the past several months of hearings and legal battles -- except with the possibility of actual impeachment votes.

Her brief statement, delivered without dramatic flourish but in the framework of a constitutional crisis, capped a frenetic weeklong stretch on Capitol Hill as details of a classified whistleblower complaint about Trump burst into the open and momentum shifted toward an impeachment probe.

For months, the Democratic leader has tried calming the push for impeachment, saying the House must investigate the facts and let the public decide. The new drive was led by a group of moderate Democratic lawmakers from political swing districts, many of them with national security backgrounds and serving in Congress for the first time. The freshmen, who largely represent districts previously held by Republicans where Trump is popular, risk their own reelections but say they could no longer stand idle. Amplifying their call were longtime leaders, including Rep. John Lewis of Georgia, the civil rights icon often considered the conscience of House Democrats.

"Now is the time to act," said Lewis, in an address to the House. "To delay or to do otherwise would betray the foundation of our democracy."

At issue are Trump's actions with Ukraine. In a summer phone call with Ukrainian President Volodymyr Zelenskiy, he is said to have asked for help investigating former Vice-President Biden and his son Hunter. In the days before the call, Trump ordered advisers to freeze $400 million in military aid for Ukraine -- prompting speculation that he was holding out the money as leverage for information on the Bidens. Trump has denied that charge, but acknowledged he blocked the funds, later released.

Biden said Tuesday, before Pelosi's announcement, that if Trump doesn't co-operate with lawmakers' demands for documents and testimony in its investigations the president "will leave Congress ... with no choice but to initiate impeachment." He said that would be a tragedy of Trump's "own making."

The Trump-Ukraine phone call is part of the whistleblower's complaint, though the administration has blocked Congress from getting other details of the report, citing presidential privilege. Trump has authorized the release of a transcript of the call, which is to be made public Wednesday.

"You will see it was a very friendly and totally appropriate call," Trump said.

The whistleblower's complaint was being reviewed for classified material and could go to Congress by Thursday, according to a person familiar with the issue who was not authorized to discuss it publicly and spoke on condition of anonymity.

Trump has sought to implicate Biden and his son in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading the Obama administration's diplomatic dealings with Kyiv. Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice-president or his son.

While the possibility of impeachment has hung over Trump for many months, the likelihood of a probe had faded after special counsel Robert Mueller's Trump-Russia investigation ended without a clear directive for lawmakers.

Since then, the House committees have revisited aspects of the Mueller probe while also launching new inquiries into Trump's businesses and various administration scandals that all seemed likely to drag on for months.

But details of Trump's dealings with Ukraine prompted Democrats to quickly shift course. By the time Pelosi addressed the nation Tuesday, about two-thirds of House Democrats had announced moving toward impeachment probes.

The burden will probably now shift to Democrats to make the case to a scandal-weary public. In a highly polarized Congress, an impeachment inquiry could simply showcase how clearly two sides can disagree when shown the same evidence rather than approach consensus.

Building toward this moment, the president has repeatedly been stonewalling requests for documents and witness interviews in the variety of ongoing investigations.

After Pelosi's Tuesday announcement, the president and his campaign team quickly released a series of tweets attacking Democrats, including a video of presidential critics like the speaker and Rep. Ilhan Omar discussing impeachment. It concluded: "While Democrats 'Sole Focus' is fighting Trump, President Trump is fighting for you."

Senate Majority Leader Mitch McConnell said Pelosi's well-known "efforts to restrain her far-left conference have finally crumbled."

Pelosi has for months resisted calls for impeachment from her restive caucus, warning that it would backfire against the party unless there was a groundswell of public support. That groundswell hasn't occurred, but some of the more centrist lawmakers are facing new pressure back home for not having acted on impeachment.

On Wednesday, the Democratic-controlled House is expected to consider a symbolic but still notable resolution insisting the Trump administration turn over to Congress the whistleblower's complaint. The Republican-led Senate, in a rare bipartisan moment, unanimously approved a similar resolution Tuesday.

The lawyer for the whistleblower, who is still anonymous, released a statement saying he had asked Trump's director of national intelligence to turn over the complaint to House committees and asking guidance to permit the whistleblower to meet with lawmakers.

Pelosi suggested that this new episode -- examining whether a president abused his power for personal political gain -- would be easier to explain to Americans than some of the issues that arose during the Mueller investigation and other congressional probes.

The speaker put the matter in stark terms: "The actions of the Trump presidency revealed dishonourable facts of the president's betrayal of his oath of office, betrayal of his national security and betrayal of the integrity of our elections."

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Associated Press writers Julie Pace, Laurie Kellman, Andrew Taylor, Eric Tucker and Zeke Miller in Washington and Jonathan Lemire and Deb Riechmann in New York contributed to this report.

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

PRESIDENTIAL HARASSMENT! — Donald J. Trump (@realDonaldTrump) September 24, 2019