WASHINGTON — A crucial cache of evidence in hand, House Democrats moved quickly on Thursday with an impeachment inquiry they said would be focused tightly on President Trump’s dealings with Ukraine, using an incendiary whistle-blower complaint as a road map for their investigation.

The complaint landed like a bombshell on Capitol Hill on Thursday morning after its release by the House Intelligence Committee, and Democrats quickly seized on its narrative of allegations against Mr. Trump — chock-full of potentially damning detail, intriguing threads and characters who could become witnesses in the nascent inquiry — as an outline for their work.

After months of plodding investigating to determine whether they had grounds to impeach Mr. Trump, Democrats were working feverishly to build a case on the Ukraine matter, with some lawmakers saying they could move within a month or six weeks, possibly drafting articles of impeachment by the end of October.

“This is a cover-up,” said Speaker Nancy Pelosi, Democrat of California, who after months of resisting the move made it clear that she was determined to follow through with a formal impeachment inquiry.

She read aloud from a portion of the document describing an attempt by White House officials to quickly “lock down” records of a phone call in which Mr. Trump asked the Ukrainian president to investigate former Vice President Joseph R. Biden Jr. The complaint detailed charges that the president “is using the power of his office to solicit interference from a foreign country in the 2020 U.S. election,” and that officials took pains to conceal evidence of that effort.

“We are at a different level of lawlessness that is clear to the American people,” Ms. Pelosi said.

The speaker said the growing impeachment case would be centered around the Ukraine matter and investigative action mostly lodged in the House Intelligence Committee, which first received and publicized the complaint.

The House Judiciary Committee, which has been leading the charge on impeachment for months, is now expected to temporarily idle the public components of its investigation of obstruction of justice and abuse of power. That inquiry focused on the findings of Robert S. Mueller III, the special counsel who investigated Russia’s election interference in 2016, and the president’s attempts to disrupt his work. Those topics could be resurrected if and when the committee drafts impeachment articles, however.

The Intelligence Committee was quickly lining up investigative targets. Speaking to reporters on Thursday, Adam B. Schiff, the committee’s chairman, said that the complaint provided a clear “road map” for congressional investigators in the coming weeks and that his committee would work through Congress’s two-week recess that begins on Friday.

At the top of his agenda, he said, is interviewing the whistle-blower and another session with the inspector general, Michael Atkinson, to better understand what preliminary investigative work he had done before notifying Congress of the complaint. But there were numerous leads tucked into the complaint, and fresh questions about the role of Rudolph W. Giuliani, Mr. Trump’s personal lawyer, and William P. Barr, the attorney general, in the matter.

“We need to look into the allegation that this may not be the only communication of a potentially corrupt character that was shielded by this classified information computer system abused for that purpose,” Mr. Schiff said.

“We want to know what role Rudy Giuliani had in all of this,” he added. “We want to know what role Bill Barr had in any of this. We want to know what Ukraine understood was expected of them before they even had this July phone call with the president of the United States.”

The complaint became public just minutes before the intelligence panel prepared to hear testimony from Joseph Maguire, the acting director of national intelligence, on why he had delayed sharing it with Congress for nearly a month over the recommendation of the intelligence community’s inspector general. It was the second time in two days that their nascent impeachment inquiry had netted a significant tranche of potential evidence — a forceful reminder of the House’s newfound leverage after months of stonewalling from the White House.

Facing tough questioning from Republicans and Democrats, Mr. Maguire defended both the whistle-blower’s actions and his handling of the case, which he called “urgent and important.”

Though it lacked any blockbuster admissions, the hearing gave Democrats new leads to pursue about who Mr. Maguire interacted with in the executive branch as he sought to determine whether he should disclose the complaint to Congress. And it provided a venue to try to establish the whistle-blower’s credibility, even as Republicans sought repeatedly to dismiss his complaint.

The whistle-blower, Mr. Maguire repeatedly said, had “acted in good faith” and “followed the law every step of the way.”

But there were limits. As Democrats repeatedly pressed him to endorse their investigation, Mr. Maguire bristled.

“My responsibility was to get you the whistle-blower letter and get the other information released. I have done my duty,” he said. Whether to investigate further “is on the shoulders of the legislative branch and this committee.”

Democrats already have outstanding requests to the State Department and the White House for all records related to Mr. Trump’s call with the Ukrainian leader, his decision to withhold $391 million in security aid from Ukraine at the same time he was pushing for an investigation of Mr. Biden and attempts to influence Ukrainian policy by Mr. Giuliani. If the requests are not fulfilled voluntarily, they could issue subpoenas compelling delivery of the material as soon as Friday.

Those demands may test whether the White House’s decisions to turn over the whistle-blower complaint and, earlier, a summary of Mr. Trump’s call with the Ukrainian leader were aberrations or mark a new turn toward cooperation with the House investigations.

“We should be focused and not overthink this,” said Representative Eric Swalwell, Democrat of California and a member of the intelligence panel. “The guy has copped to this. If you are running an investigation, it is usually made a lot easier when the person admits to the crime. You can cross a lot of witnesses off the list that you might otherwise talk to.”

As the revelations piled up on Thursday, few members of the president’s party spoke out against him and many of the most prominent Republican lawmakers offered an energetic defense.

Representative Kevin McCarthy of California, the Republican leader, complained that the whistle-blower “has no primary sources.”

In the hearing with Mr. Maguire, Representative Devin Nunes of California, the top Republican on the intelligence panel, accused Democrats of starting another “information warfare operation against the president,” just like when they fanned the flames of what he called the “Russia hoax.”

“They don’t want answers,” Mr. Nunes said. “They want a public spectacle.”

Some Republicans, however, appeared to be growing weary of defending Mr. Trump against the torrent of new accusations.

Representative Michael R. Turner, Republican of Ohio and another Intelligence Committee member, criticized Mr. Schiff for an opening statement in which he exaggerated Mr. Trump’s conversation with the Ukrainian leader for dramatic effect. But he also leveled a sharp critique at the president, based on the transcript of his call with the Ukrainian leader that was released on Wednesday: “Concerning that conversation, I want to say to the president: ‘This is not O.K. That conversation is not O.K.’”

Catie Edmondson contributed reporting.