ASSOCIATED PRESS, LISA MASCARO, MARY CLARE JALONICK, and MICHAEL BALSAMO

House Speaker Nancy Pelosi announced Tuesday that the House of Representatives will take the rare step of launching 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 re-election. Pelosi said such actions would mark a “betrayal of his oath of office” and declared: “No one is above the law.”

Only three of Trump’s predecessors underwent similar proceedings: Andrew Johnson and Bill Clinton, who were acquitted after trials in the Senate, and Richard Nixon, who resigned to avoid being impeached in connection with the Watergate scandal.

The rarely used procedure is spelled out in Article II, Section 4 of the Constitution, which stipulates that the president and other officers of government “shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors.”

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 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 specter of impeachment led by the opposition party will bolster rather than diminish his political support.

Meeting with world leaders at the United Nations, he previewed his defense in an all-caps tweet: “PRESIDENTIAL HARASSMENT!”

Pelosi’s 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.

Rep. Abigail Spanberger, D-7th, sensed sentiment in her politically mixed Virginia district “pivot” from a “general pit of confusion” over the Mueller report to clarity.

“I saw a shift on the ground in my district,” said Spanberger, whose district includes portions of Chesterfield and Henrico counties. “On the ground there was a change in the conversation. This is something new. It’s a new phase. This is a new type of allegation.”

Virginia Rep. Elaine Luria, D-2nd, said, “It’s crystal clear. If this isn’t impeachable, what is?”

After Pelosi told the caucus she was moving forward with the inquiry, Luria said lawmakers clapped but the mood was sober.

“I didn’t come to Washington wanting to impeach the president,” said Luria, a former Navy commander who served in the military for 20 years. “It’s obviously a very sad day for our country.”

Luria and Spanberger won races in districts that previously voted for Trump, part of a wave of moderate Democrats who helped flip the House in 2018. They both resisted earlier calls for impeachment by more progressive members of the Democratic caucus.

But over the weekend, as news broke about Trump’s communications and actions toward Ukraine, Luria and Spanberger joined five fellow freshman lawmakers with backgrounds in national security to craft an op-ed in The Washington Post to announce their support for impeachment. It posted late Monday.

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 Biden’s 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 cooperate with lawmakers’ demands for documents and testimony in their 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 on 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 .

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 their willingness to move toward impeachment probes.

The burden will likely 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 announcement Tuesday, 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.”

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.

On Wednesday, the House is expected to consider a symbolic but still notable resolution insisting the Trump administration turn over to Congress the whistleblower’s complaint. The Senate, in a rare bipartisan moment, 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.