WASHINGTON — House Democrats hurtled on Tuesday toward a consequential set of decisions about the potential impeachment of President Trump, weighing a course that could reshape his presidency amid starting allegations that he sought to enlist a foreign power to aid him politically.
House Speaker Nancy Pelosi, Democrat of California, who has stubbornly resisted a rush to impeachment, appeared to be rapidly changing course, as lawmakers from every corner of her caucus lined up in favor of filing formal charges against Mr. Trump if the allegations are proven true, or if his administration continues to stonewall attempts by Congress to investigate them.
One possibility was the formation of a special committee — reminiscent of the one created in 1973 to investigate the Watergate scandal — to look into the president’s dealings with Ukraine and potentially lay the groundwork for articles of impeachment based on the findings.
Ms. Pelosi planned a meeting Tuesday afternoon to coordinate strategy with the six committee chairmen who have led the investigations of Mr. Trump, followed by a broader closed-door meeting of all of the chamber’s Democrats to brief them and gauge their mood in light of the changed circumstances.
Calls for impeachment have mounted, with a growing list of vulnerable moderates — until now the chief skeptics of the move — stating that they believed articles of impeachment would be the only recourse if reports about attempts by Mr. Trump and his personal lawyer to push Ukraine to investigate former Vice President Joseph R. Biden Jr. and his son were true.
“The first responsibility of the president of the United States is to keep our country safe, but it has become clear that our president has placed his personal interests above the national security of our nation,” Representative Antonio Delgado, Democrat of New York and one of the party’s most politically vulnerable freshman moderates, wrote on Tuesday. “I believe articles of impeachment are warranted.”
Progressives, including Senator Elizabeth Warren of Massachusetts, a leading Democratic candidate for president, demanded even faster action. “It must start today,” she said of impeachment.
Mr. Trump, in New York for his second day of diplomatic meetings at the United Nations, dismissed the effort as a desperate political ploy by Democrats, and continued to maintain he had done nothing wrong.
“I think it’s ridiculous,” Mr. Trump told reporters. “It’s a witch hunt. I’m leading in the polls. They have no idea how they stop me. The only way they can try is through impeachment. This has never happened to a president before.”
House Republicans’ campaign arm blasted out a statement predicting Democrats would be ensuring the end of their House majority if they followed through.
The shift in outlook among Democratic lawmakers has been rapid, and could yet still turn away from impeachment if exculpatory evidence comes to light. The developments that have turned the tide began less that two weeks ago, when Representative Adam B. Schiff of California, the Intelligence Committee chairman, first revealed the existence of a secretive whistle-blower complaint that the intelligence community’s internal watchdog had deemed “urgent” and credible but the Trump administration had refused to share with Congress.
That complaint remains secret, and lawmakers are fighting to see it, but news reports have established that the complaint was related, at least in part, to a July call between Mr. Trump and President Volodymyr Zelensky of Ukraine in which Mr. Trump pressed the foreign leader to investigate Mr. Biden and his son, Hunter, for corruption.
Just days earlier, Mr. Trump had ordered his staff to freeze more than $391 million in aid to Ukraine. While Mr. Trump denies having explicitly linked the two issues, lawmakers believe they are connected and have demanded documentation that could clarify the situation.
The House Judiciary Committee has been conducting what it has called an impeachment investigation focused on the findings of the special counsel, Robert S. Mueller III, as well as allegations that Mr. Trump may be illegally profiting from spending by state and foreign governments, and other matters. But that inquiry never got the imprimatur of a full House vote or the full rhetorical backing of the speaker, as Democrats remained divided about the wisdom and political implications of impeaching a president without broader public support.