"The actions of the Trump presidency revealed the dishonorable fact of the president's betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections," Pelosi said. "Therefore, today I'm announcing the House of Representatives is moving forward with an official impeachment inquiry."
Her announcement came as questions mounted related to a formal complaint filed by a whistleblower concerning a phone call Trump had with Ukraine’s president in July and whether Trump had pressured him to investigate former Vice President Joe Biden.
Trump has said he asked Ukraine’s President Volodymyr Zelensky about investigating Biden and his son Hunter as part of corruption in Ukraine Trump has alleged, but he has denied he tied aid to the country to whether Ukraine’s president went ahead with the probe. The administration put a temporary hold on aid to Ukraine shortly before the call.
"When you see the call, when you see the readout of the call -- which I assume you'll see at some point -- you'll understand," Trump told reporters Tuesday morning as he arrived at the U.N. General Assembly in New York, before his tweet about releasing the call transcript. "That call was perfect. It couldn't have been nicer."
Since reports of the call surfaced, Trump has offered two different explanations for why he put a pause on aid money being sent to Ukraine. He's stated concerns about corruption in the country, and he's also suggested that he paused the funding because he believes other countries should be supplying more money to aid Ukraine.
Congress also is seeking the complaint filed by the whistleblower. Democrats have been blocked from getting it despite a law they say requires the intelligence community inspector general to send it to Congress within seven days if he deems the complaint “credible."
Tuesday evening, two sources close to the president told ABC News that the White House is considering releasing the complaint filed by the whistleblower. The complaint, which is classified, is undergoing a declassification process, according to sources familiar with the matter.
Democrats also want a copy of the Intelligence Community Inspector General Michael Atkinson's report on the complaint that he sent to Director of National Intelligence Joseph Maguire. Maguire, who blocked Atkinson from sending the complaint to Congress, is supposed to testify in public before the House Intelligence Committee Thursday.
The Democrat who chairs the House Intelligence Committee, Rep. Adam Schiff, D-Calif., tweeted Tuesday afternoon that the whistleblower wants to speak to his committee.
(MORE: Top intelligence official to brief Congress on whistleblower complaint)
Meanwhile, for the last several months, the House Judiciary Committee has continued to follow up on the former special counsel Robert Mueller’s report, weighing more formal impeachment proceedings.
Five other House committees are investigating various aspects of the Trump presidency, from government and military spending at his private properties, to elements of his finances and personal tax returns.
While support for impeaching Trump in the wake of the Mueller investigation has steadily grown, especially among progressive Democrats, the new reports that the president repeatedly pressed the Ukrainian leader to investigate a potential 2020 Democratic opponent and his son’s business dealings - while instructing top aides to freeze aid to Ukraine approved by Congress -- has forced a reckoning on Capitol Hill.
A number of House Democrats, including several more moderate members, came out Tuesday in favor of impeachment for the first time, with some arguing the president betrayed the obligations of his office in his conversation with the Ukraine president.
"It is clear to me that he has betrayed the public trust and abandoned his obligations to the Constitution by elevating his own interests over the national interest," Rep. Elaine Luria, D-Va., wrote in a statement Tuesday. "Congress must investigate and use the full extent of its powers to check these alleged abuses of presidential power. The House must move forward with impeachment."
As of Tuesday, at least 188 Democrats had stated that they were in favor of an impeachment inquiry.
Until now, Pelosi has been cautious to move forward on impeachment but was set to make an announcement after a meeting with all House Democrats Tuesday afternoon.
When Pelosi to the speaker’s gavel back in January -- after Democrats reclaimed the majority in the House -- she urged her caucus to wait to make any decision until the full Mueller report on Russian interference in the 2016 election and possible obstruction of justice by the president was completed.
"I don't think we should go down that path, because it divides the country," Pelosi told the Washington Post in March. "And he's just not worth it."
After the Mueller report’s release, Pelosi again expressed reluctance to pursue the measure, emphasizing the importance of fact-finding and investigating.
"If the facts, the path of fact-finding takes us there, we have no choice," Pelosi said at a summit. "But we're not there yet."
Over the weekend, Pelosi sent a letter to all members of the House which seemed to signal a shift in her position. In the letter, she called for the administration to allow Congress view the whistleblower complaint, and accused the administration of "endangering our national security."
"If the Administration persists in blocking this whistleblower 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," Pelosi wrote.
ABC News' Katherine Faulders and John Santucci contributed to this report.