WASHINGTON — Speaker Nancy Pelosi and President Trump were discussing gun violence over the telephone Tuesday morning when the president abruptly changed the topic to an intelligence community whistle-blower complaint that had Democrats talking about impeachment.
Ms. Pelosi stopped him short.
“Mr. President,” she declared, according to a person familiar with the conversation, “you have come into my wheelhouse.”
The remark was a reference to Ms. Pelosi’s quarter-century of experience with intelligence matters in Congress, an aspect of her biography that played a central role in her decision on Tuesday to open a formal impeachment inquiry into Mr. Trump.
The move is her most consequential act as speaker, and one that could shape the future of the Trump presidency, the 2020 election and the nation. It is the latest chapter in her fraught relationship with the president, and a stark turnabout for Ms. Pelosi, who until days ago had been deeply resistant to impeaching Mr. Trump despite the deep disgust she has expressed for him and his conduct.
For Ms. Pelosi, the intelligence and national security implications of the latest allegations against Mr. Trump helped turn the tide.
Long before she was speaker, Ms. Pelosi served as the top Democrat on the House Intelligence Committee, overseeing the secretive workings of America’s national security apparatus and helping to draft the law that governs how intelligence officials file whistle-blower complaints, and how that information is shared with Congress.
She saw the latest allegations against Mr. Trump — that he pressured the Ukrainian president to investigate a leading political rival, then worked to bury an intelligence whistle-blower’s complaint detailing the effort — as an egregious perversion of the process.
Ms. Pelosi has always been a confounding figure for Mr. Trump, as a powerful woman unafraid to confront him to his face, and one of the few people in Washington whom he regards as an equal. Her televised put-down of the president during last year’s government shutdown — “Please do not characterize the strength that I bring,” she told Mr. Trump, after he tried to undercut her during a meeting in the Oval Office — made their relationship an instant source of fascination in Washington, even before she officially became speaker.
Now, at a deeply divisive moment in American politics, the speaker is confronting the president yet again, by drawing on something Mr. Trump does not have: an intimate knowledge of the intelligence community, gleaned from 10 years on the panel and 15 more as an ex officio member, by dint of her position in leadership.
She comes at it with a bring-it-on attitude, telling colleagues that she is uniquely positioned to confront the president on matters of intelligence and national security.
“Some of you know, some of you don’t,” Ms. Pelosi said of her 25 years on the intelligence panel, during a closed-door meeting of House Democrats Tuesday night before announcing her impeachment decision, according to a Democratic aide who was in the room. “Nobody has ever served in leadership who had intelligence experience and exposure.”
Republicans charge that Ms. Pelosi is abusing the impeachment process.
“I just watched the speaker yesterday demean the office of the speakership,” Representative Kevin McCarthy, Republican of California and the minority leader, said on Wednesday, calling it “a dark day for the rule of law” and “a dark day for national security.”
He pronounced himself “disgusted by the action of the speaker of the House and the majority party, because they did not like the outcome of the 2016 election.”
For months, as the left has clamored for impeachment proceedings, Ms. Pelosi has pushed back, urging caution. She has been guided by a deep conviction that the process would be bitterly divisive for the country and would not only fail to remove Mr. Trump, but potentially boost him politically even as it harmed moderate Democrats in Republican-leaning districts and, by extension, the party’s chances of keeping the House majority.
But the president’s admission that he urged the leader of Ukraine to investigate former Vice President Joseph R. Biden Jr., a leading contender for the Democratic presidential nomination, and his administration’s refusal to give Congress a whistle-blower complaint about it, put her over the edge.
In 1998, she helped write a law protecting intelligence community whistle-blowers — the same law that Democrats argue that Mr. Trump is flouting.
“She takes very, very seriously the fact that we have a clear statute, that she was involved in drafting or getting passed, that says you shall turn over this whistle-blower complaint,” said Representative Tom Malinowski, Democrat of New Jersey. “That is as clear as day. What I’m hearing is that’s a big factor in the decision that she’s come to.”
Ms. Pelosi, Democrat of California, joined the intelligence panel in 1993, just six years after she won a special election to fill a vacancy created by the death of her predecessor, Sala Burton. As an ambitious woman eager to rise up in the ranks, she felt she needed to gain foreign policy credentials. She saw the Intelligence Committee as a path into leadership.
“She would often talk about trying to ascend the leadership and the glass ceiling that was imposed on women — particularly in the areas of foreign policy and national security, as if those areas were the exclusive domain of men,” said Steve Israel, a former congressman from New York who served in leadership with Ms. Pelosi.
“She always took great pride in shattering that glass ceiling as someone who not only got into the leadership of the House, but played a leading role on the Intelligence Committee,” Mr. Israel said.
She spent 10 years on the panel, eventually rising to the role of ranking member. Once she became Democratic leader in 2003, she left the committee’s roster but continued in an ex officio capacity. That makes her a member of the so-called Gang of Eight — the bipartisan leaders of the House and the Senate, and their Intelligence Committees. The gang had frequent meetings with Presidents George W. Bush and Barack Obama, less so with Mr. Trump. Stacy Kerr, a former senior adviser to Ms. Pelosi, says that exposure may have shaped the speaker’s approach to this current president: “I think she maybe learned the art of discretion is that your power is what you know. Your power is not always that you articulate what you know.”
Ms. Pelosi’s tenure on the panel unfolded in a very different era, when the Intelligence Committee was known for working in a bipartisan way. She was ranking Democrat in 2001, at the time of the Sept. 11 attacks, and was instrumental in creating a joint House-Senate committee to investigate the intelligence failures that led to them.
In 2003, when Mr. Bush sought authorization to go to war with Iraq, Ms. Pelosi voted against it, saying repeatedly that she did not believe the intelligence supported Mr. Bush’s contention that Iraq possessed weapons of mass destruction. She turned out to be correct — a story she recounts in her 2008 autobiography, “Know Your Power.”
“As the senior Democrat on the Intelligence Committee at the time of the Iraq vote, I said to the press when I voted ‘No’ on the war that the intelligence did not support the imminent threat the administration was claiming,’’ Ms. Pelosi wrote. “And the press asked, ‘Are you calling the president a liar?’ ‘I’m stating a fact,’ I replied.”
But Ms. Pelosi also drew criticism for failing to object to the Bush administration’s post-9/11 secret eavesdropping program, which she had been informed of as Democratic leader. She said at the time that she expressed her concerns about the program to the Bush administration in a letter, but could not make the letter public because it was classified.
Handling of classified information was at the heart of the congressional debate over the 1998 whistle-blower law, which played out against the backdrop of accusations that members of Congress were leaking classified material.
The chairman of the intelligence panel at the time, Porter Goss, Republican of Florida, who later went on to become Mr. Bush’s C.I.A. director, convened two hearings with the aim of creating a system that would enable intelligence officials to bring complaints to Congress in a professional way, without having to fear they would be charged with revealing classified information.
At one of those hearings, in June 1998, Ms. Pelosi expressed her concern that “a person with conscience with information” would be “punished and have his security clearance removed because he revealed classified information to a member” of the Intelligence Committee. The proposed legislation passed and President Bill Clinton signed it into law. At Tuesday night’s meeting, she boasted about it.
“I know what the intent of the law was,” she said, according to the aide. “And what the intent of the law was to secure, in cases like this, to secure our intelligence and to protect our whistle-blowers.”