WASHINGTON, D.C. — Speaker Nancy Pelosi announced Tuesday that the House will pursue an impeachment inquiry of President Donald Trump, setting the stage for a monumental clash between Congress and the president a year away from the 2020 election, according to Democratic sources familiar with her plans.

With more than a dozen moderate House Democrats joining the long list of those demanding an impeachment inquiry, Pelosi, D-Calif., is responding to mounting pressure from her caucus.

It came as Trump earlier in the day acknowledged for the first time that he had held up aid to Ukraine shortly before speaking with that country’s president, Volodymyr Zelensky, and urging the foreign leader to investigate former Vice President Joe Biden, a possible Trump opponent in the 2020 presidential race.

Those actions reportedly triggered an unidentified whistleblower in the intelligence community to write a complaint about the president. The Office of the Director of National Intelligence has refused to share the complaint with Congress, triggering a battle with Democratic lawmakers.

Rep. Adam Schiff, D-Calif., tweeted Tuesday that the whistleblower had expressed interest in speaking with the House Intelligence Committee, perhaps as soon as this week.

“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 as to how to do so. We’re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week,” Schiff wrote.

On Tuesday afternoon, Trump announced he had ordered the release on Wednesday of the transcript of his July 25 call with Zelensky.

Trump has claimed his actions were appropriate. First, he said he was primarily concerned about ensuring the U.S. funds were not being misused. On Tuesday, he said he withheld the aid out of frustration that European nations were not contributing their fair share of aid to the country.

Democrats want to know whether the president held hostage the Ukrainian aid that Congress had approved in order to pressure Zelensky to meddle in the 2020 U.S. election. The aid was released earlier this month.

Pelosi was set to gather Democrats behind closed doors in the Capitol on Tuesday afternoon to discuss how Congress should proceed, and make her announcement after that meeting.

During an appearance at the Atlantic Festival on Tuesday, Pelosi said she believes the administration is breaking the law.

“The DNI is at present time breaking the law at the direction of the administration,” Pelosi said.

Six freshmen House Democrats with national security backgrounds warned in a Washington Post op-ed posted late Monday that Trump’s actions would be impeachable offenses. It was not a call for impeachment, but the identities of authors — they all won in Trump districts — set off a cascade.

Since then, at least a dozen other Democrats who had been holdouts against impeachment —including Reps. Lizzie Fletcher of Texas, Antonio Delgado of New York — reversed course and came out with their support for an inquiry. Together, they represent some of the most Trump-leaning districts in the country, suggesting they do not believe there will be a political price to pay for beginning an impeachment inquiry.

Several lawmakers who are close allies of the speaker, including Reps. Rosa DeLauro, D-Conn., and Debbie Dingell, D-Mich., indicated that they, too, support the call. More than two-thirds of House Democrats now support impeachment.

Congressman Derek Kilmer, a Democrat who represents Grays Harbor and most of the Olympic Peninsula, has supported an impeachment inquiry since late July.

His cited the Mueller Report when he called for an inquiry, saying the level of foreign interference in elections and evidence of obstruction of justice overcame his concerns that impeachment will further divide the country.

“The president’s open acceptance of support from Russians was undeniably disruptive to the integrity of our democracy. We are now learning that the Russians penetrated election systems in all 50 states. This ongoing threat is real, and it is the duty of both parties to protect our democracy from a similar assault in 2020.

“… In my view, the incidents of obstruction and of foreign interference cited in the Mueller report should not be dismissed based on politics, party biases, or the fear of some predicted outcome,” Kilmer said.

“The time to begin impeachment proceedings against this president has come,” said Rep. John Lewis, D-Ga., the civil rights leader whose opinion weighs heavily with Democrats and who is often aligned with Pelosi.

At Tuesday’s meeting, Democrats were expected to discuss next steps. Progressives, who have long backed an aggressive inquiry of the president, are expected to push for an immediate inquiry with a strict timetable for quick action.

Others are expected to push for a more nuanced approach, including an emphasis on obtaining a whistleblower’s complaint related to the president’s actions regarding Ukraine, including the phone call.

Among other things, Democrats must decide how the inquiry will proceed, including which Democrats would lead a potential special committee.

Also under discussion will be a likely House resolution condemning the president’s discussion with the Ukrainian leader.

Pelosi has long opposed starting impeachment proceedings, arguing that without overwhelming evidence and bipartisan support, it would be fruitless. It would also put moderate Democrats in Trump-leaning districts in a difficult position to defend back home.

“The speaker is in perpetual conversation with her caucus,” said Danny Weiss, Pelosi’s former chief of staff. “This is definitely a turning point for the caucus and for the speaker.”

Still, the political dynamic that has worried House Democrats since the beginning of the year has not changed. Even if the House is able to impeach the president, the Republican-controlled Senate is almost certainly not going to convict him. Trump would remain in office and have a Senate acquittal run on.

No Senate Republican has publicly suggested that he or she views the Ukraine events as impeachable, although Sens. Mitt Romney, R-Utah, and Susan Collins, R-Maine, have gone the furthest among any Senate Republican, demanding that the complaint be turned over to Congress.