By Nicholas Fandos, Jonathan Martin and Maggie Haberman, New York Times–

President Trump acknowledged on Sunday that he raised corruption accusations against former Vice President Joseph R. Biden Jr. during a phone call with Ukraine’s leader, a stunning admission as pressure mounted on Democrats to impeach Mr. Trump over allegations he leaned on a foreign government to help damage a political rival.

In public and in private, many Democrats said the evidence that has emerged in recent days indicating that Mr. Trump pushed the Ukrainian government to investigate Mr. Biden, and his administration’s stonewalling of attempts by Congress to learn more, were changing their calculations about whether to charge him with articles of impeachment.

The influential chairman of the House Intelligence Committee, who has resisted such action, said the House might now have “crossed the Rubicon” in light of the new disclosures, and the administration’s withholding of a related whistle-blower complaint. A group of moderate freshman lawmakers who had been opposed to an impeachment inquiry said they were considering changing course, while other Democrats who had reluctantly supported one amplified their calls. Progressives, meanwhile, sharpened their criticisms of the party’s leadership for failing to act.

The fast-moving developments prompted Speaker Nancy Pelosi to level a warning of her own to the White House: Turn over the secret whistle-blower complaint by Thursday, or face a serious escalation from Congress.

In a letter to House Democrats, Ms. Pelosi never mentioned the word “impeachment,” but her message hinted at that possibility.

“If the administration persists in blocking this whistle-blower from disclosing to Congress a serious possible breach of constitutional duties by the president, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation,” Ms. Pelosi, Democrat of California, wrote in the letter.

The allegations center on whether Mr. Trump pressured Ukraine’s newly elected leader to take action to hurt Mr. Biden’s election bid, implicitly or explicitly, at a vulnerable moment for the former Soviet republic, possibly using United States military aid as leverage. Ukraine has been fighting Russian-backed separatists, and the Trump administration had temporarily been withholding a $250 million package of military funding. There have been no indications to this point, however, that Mr. Trump mentioned the aid money on the call.

Mr. Trump showed no sign of contrition on Sunday, telling aides that Democrats were overplaying their hand on a matter voters would dismiss. Publicly, he worked to focus attention not on his own actions, but on Mr. Biden’s.

Speaking to reporters, the president defended his July phone call with President Volodymyr Zelensky of Ukraine as entirely appropriate, and stopped short of directly confirming news reports about what was discussed. But he acknowledged that he had discussed Mr. Biden during the call and accused the former vice president of corruption tied to his son Hunter’s business activities in the former Soviet republic.

“The conversation I had was largely congratulatory, with largely corruption, all of the corruption taking place and largely the fact that we don’t want our people like Vice President Biden and his son creating to the corruption already in the Ukraine,” Mr. Trump told reporters before leaving for a trip to Texas and Ohio.

It is still far from clear that the latest scandal surrounding Mr. Trump’s conduct will lead Ms. Pelosi or other top Democrats to bless full impeachment proceedings and a vote. The House Judiciary Committee is already investigating whether to recommend articles of impeachment against Mr. Trump over other matters, but Ms. Pelosi has consistently questioned the strength of the case.

Proponents of impeachment have repeatedly pointed to damaging revelations — including several instances of possible obstruction of justice by Mr. Trump detailed by the special counsel investigating Russia’s interference in the 2016 election — that they believe warrant seeking Mr. Trump’s removal. But they have run into resistance or indifference from their colleagues and the general public, in part because any impeachment proceeding could end in an acquittal by the Republican-controlled Senate.

On Sunday, the pattern appeared to be holding, with the vast majority of Republican lawmakers refraining from comment about the latest allegations against Mr. Trump. A few prominent lawmakers suggested, however, that the White House should disclose the contents of the phone call with Mr. Zelensky.

“I’m hoping the president can share, in an appropriate way, information to deal with the drama around the phone call,” said Senator Lindsey Graham of South Carolina. “I think it would be good for the country if we could deal with it.”

Senator Mitt Romney of Utah, the Republican presidential nominee in 2012, was more critical, deeming it “critical for the facts to come out” and saying, “If the president asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme.”

At the same time, interviews with more than a dozen Democratic lawmakers this weekend made clear that they believed the latest allegations had the potential to be singularly incriminating, with the potential to advance the impeachment drive just as it appeared to be losing steam. Not only do the allegations suggest that Mr. Trump was using the power of his office to extract political gains from a foreign power, they argued, but his administration is actively trying once again to prevent Congress from finding out what happened.

“I don’t want to do any more to contribute to the divisiveness in the country, but my biggest responsibility as an elected official is to protect our national security and Constitution,” said Representative Debbie Dingell of Michigan, adding that it is “becoming more and more difficult” for Democrats to avoid an all-out impeachment inquiry.

Several first-term lawmakers who had opposed impeachment conferred privately over the weekend to discuss announcing support for an inquiry, potentially jointly, after a hearing scheduled for Thursday with the acting national intelligence director, according to Democratic officials familiar with the conversations. A handful of them declined to speak on the record over the weekend, with some still reluctant to go public and others looking for cues from Ms. Pelosi and their freshman colleagues.

Representative Tom Malinowski, a New Jersey freshman who has supported an inquiry, said the fresh revelations made it clear that Congress must move more decisively.

“There are lines being crossed right now that I fear will be erased if the House does not take strong action to assert them, to defend them,” he said in an interview. “If all we do is leave it up to the American people to get rid of him, we have not upheld the rule of law, we have not set a precedent that this behavior is utterly out of bounds.”

The Intelligence Committee chairman, Representative Adam B. Schiff of California, said Sunday morning that the accumulating evidence of wrongdoing, and of a presidential cover-up unfolding in real time, left the House with few other options. Mr. Schiff spoke with Ms. Pelosi before making his remarks to coordinate their statements, two people familiar with their conversation said, a sign that the speaker may be more comfortable moving toward a direct discussion of impeachment.

“I have been very reluctant to go down the path of impeachment,” Mr. Schiff said on CNN. “But if the president is essentially withholding military aid at the same time he is trying to browbeat a foreign leader into doing something illicit, providing dirt on his opponent during a presidential campaign, then that may be the only remedy that is coequal to the evil that that conduct represents.”

Mr. Schiff first brought the existence of the whistle-blower complaint to light a little more than a week ago, and has been the party’s lead negotiator with the acting director of national intelligence, who has refused to turn it over to Congress.

Progressives in Congress have watched the stonewalling with seething frustration, and in recent days, they have begun to openly second-guess Ms. Pelosi’s go-slow approach.

“At this point, the bigger national scandal isn’t the president’s lawbreaking behavior — it is the Democratic Party’s refusal to impeach him for it,” Representative Alexandria Ocasio-Cortez, Democrat of New York, who commands considerable influence among progressives, wrote on Twitter late Saturday night.

Representative Pramila Jayapal, Democrat of Washington and the co-chairwoman of the Progressive Caucus, said in an interview that she was now ready to vote outright to impeach Mr. Trump, rather than simply continuing the investigation, and that she planned to make her case in public.

“There is no congressional authority anymore that we are being allowed to exercise, except the one that we have not exercised yet,” Ms. Jayapal said.

But the more crucial issue is whether Democrats from the districts Mr. Trump won or nearly lost can stomach a push to expel him.

Representative Dina Titus of Nevada said once a transcript is made public of Mr. Trump pressuring Mr. Zelensky, she doubted that even Democrats from competitive seats could continue to resist impeachment.

“Once that comes out,” said Ms. Titus, an impeachment proponent, “I don’t see how they can fight it any longer.”

Strikingly, some traditionally cautious veteran Democrats said the party might have no choice but to move toward impeachment. They believe that Senate Republicans, who are clinging to their majority of 53 seats, would pay a political price for protecting Mr. Trump if they voted to exonerate him in the face of damning evidence of malfeasance and a House vote to impeach.

“They’ve got to take a second look” at impeachment, Terry McAuliffe, the former Virginia governor and national party chairman, who is an ally of Ms. Pelosi, said of fellow Democrats. He predicted that the latest revelations would “push some of our folks over.”

James Carville, the longtime Democratic strategist, said he had opposed impeachment, but now thinks the House should move “quick and clean” after obtaining a transcript of Mr. Trump’s phone call. “Let the Senate Republicans stew,” he said.

Nicholas Fandos and Jonathan Martin reported from Washington, and Maggie Haberman from New York. Peter Baker contributed reporting from Washington.

Nicholas Fandos and Jonathan Martin reported from Washington, and Maggie Haberman from New York. Peter Baker contributed reporting from Washington.