The disclosure to Congress of the whistle-blower’s complaint, coming less than 24 hours after Speaker Nancy Pelosi announced that the House would pursue an official impeachment inquiry, underscored how rapidly things were changing now that lawmakers had pivoted to using their powers under the Constitution to weigh charges against the president.

It came as the number of House members supporting an impeachment inquiry reached 218, a critical milestone that indicates there is a majority in the House willing to at least consider drafting and voting on articles of impeachment.

Ms. Pelosi spent Wednesday largely sequestered behind closed doors, strategizing with her leadership team, top aides and a group of six committee leaders investigating Mr. Trump. She repeatedly stressed that she wanted the House to move “expeditiously” to uncover new facts about Mr. Trump’s attempts to pressure President Volodymyr Zelensky of Ukraine to investigate Joseph R. Biden Jr., one of his leading political rivals, for corruption.

As they debated how best to structure the inquiry, lawmakers made headway in obtaining documentary evidence that could constitute a crucial piece of their case. Mr. Trump’s dealings with Ukraine are at least part of the whistle-blower complaint, which the Trump administration had withheld from Congress until Wednesday afternoon and which is said to contain a detailed account of the president’s attempts to pressure a foreign power for personal political gain.

Democrats plan to make those interactions the top priority of their impeachment case, senior lawmakers and aides familiar with the speaker’s thinking said. They emphasized again and again what Ms. Pelosi has called the president’s “betrayal” of his oath, of national security and of the American electoral process.