When the moment came, shortly after 5 P.M. on Tuesday, it had been well trailed. Earlier in the afternoon, appearing at The Atlantic magazine’s festival, Nancy Pelosi, the Speaker of the House, had given the game away, saying that she would be making an announcement later. “I’ve told people, as soon as we have the facts, we’re ready,” Pelosi said, in response to a question about whether, in view of the allegation that Donald Trump pressured the President of Ukraine to investigate Joe Biden, she and her caucus were ready to go ahead with an impeachment inquiry. “Now that we have the facts, we’re ready—for later today.”

In her subsequent public statement, which she made from the Capitol Building, after meeting with the Democratic caucus, Pelosi began by running through the history of the Ukraine story, which emerged after an anonymous intelligence official filed a whistle-blower complaint that the Trump Administration has refused to turn over to Congress. Then she got to the point. “This week, the President has admitted to asking the President of Ukraine to take actions that would benefit him politically,” she said. “The actions of the Trump Presidency reveal 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 election. Therefore, today, I am announcing the House of Representatives is moving forward with an official impeachment inquiry. . . . The President must be held accountable. No one is above the law.”

Pelosi was a bit vague about what the next steps would be. Rather than announcing the formation of a select committee to pursue the impeachment investigation, she said that the six House committees that are currently investigating Trump would continue with their work “under that umbrella of impeachment inquiry.” What this means in practice wasn’t immediately clear. Jerry Nadler, the head of the House Judiciary Committee, declared weeks ago that the probe he was overseeing amounted to an impeachment inquiry. Just how different was this? Despite the unresolved questions, though, members of the Democratic caucus emerged from their meeting with Pelosi convinced that she had crossed the Rubicon. In her brief comments to reporters, Representative Ilhan Omar said the Speaker had assured them that she recognized the President had broken the law, and that they were moving quickly on impeachment.

Watch: Joseph Maguire testifies on Trump, the Ukraine call, and the whistle-blower.

In truth, Pelosi didn’t have much choice. By lunchtime on Tuesday, a hundred and sixty-three of the two hundred and thirty-five members of her caucus had expressed support for starting impeachment proceedings. Crucially, they included a large number of freshman members from blue and purple districts. A key moment came on Monday evening, when seven of these representatives—Gil Cisneros, of California; Jason Crow, of Colorado; Chrissy Houlahan, of Pennsylvania; Elaine Luria, of Virginia; Mikie Sherrill, of New Jersey; Elissa Slotkin, of Michigan; and Abigail Spanberger, of Virginia—published an op-ed in the Washington Post, in which they rehearsed the charges against Trump. He “used his position to pressure a foreign country into investigating a political opponent, and he sought to use U.S. taxpayer dollars as leverage to do it,” they wrote, concluding, “If these allegations are true, we believe these actions represent an impeachable offense.”

Once this article appeared, Pelosi’s stance against impeachment became politically untenable. For months now, she has been hanging back on the grounds that she needed to protect members like Slotkin and Sherrill, who both won suburban, Republican-held seats in 2018. Now these very representatives were undercutting her argument on the grounds that the latest allegations against Trump were too egregious to ignore.

Pelosi’s turnabout also brought her into line with the majority of her party’s Presidential candidates. Impeachment proceedings should start “today,” Elizabeth Warren said, in a Tuesday-morning tweet. Even Biden, who had previously echoed Pelosi’s reservations, had shifted his ground. Speaking in Wilmington, Delaware, hours before Pelosi made her statement, Biden said that if Trump didn’t comply with Congress’s demands for information about his call with Volodymyr Zelensky, the Ukrainian President, it “will leave Congress, in my view, no choice but to initiate impeachment.”

But, for all the apparent inevitability of Pelosi’s announcement, it still represents a gamble with the very highest stakes. Until now, she and her allies have argued that a number of conditions would need to be met for the launch of an impeachment inquiry. First, there would have to be an easy-to-understand set of facts indicating that Trump had carried out impeachable offenses. Second, there would need to be broad support in the country for impeachment. And, third, there would have to be some prospect of gaining at least some Republican support in the Senate, which would ultimately issue a verdict on the President.

How Trump Could Get Fired Evan Osnos on what it would take to cut short Trump’s Presidency.

At most, one of these conditions has been met—the first one. Even there, we still don’t have a full picture. The contents of the whistle-blower’s complaint have yet to be seen. We don’t even have the transcript of the July 25th call between Trump and Zelensky. On Tuesday afternoon, Trump tweeted from the United Nations that he would release an un-redacted version on Wednesday. If it falls short of Trump explicitly threatening to withhold American aid unless Ukraine launches an investigation into Biden and his son Hunter, Trump’s supporters will claim that it exonerates him. That would be false, of course. As Pelosi said at the Atlantic Festival, the key act, which Trump hasn’t even denied, was asking a foreign leader to dig up dirt on a political opponent. In putting his own interest before that of the country, Trump clearly betrayed his oath of office.

It remains to be seen, however, whether the Ukraine scandal will shift public opinion in favor of impeachment. In a poll from Politico/Morning Consult that was carried out before the Ukraine story broke, thirty-five per cent of respondents expressed support for the House opening impeachment proceedings, and fifty per cent said they didn’t support it. Opinion was divided sharply along party lines, with seventy per cent of Democrats supporting the idea but only thirty-one per cent of Independents and six per cent of Republicans in favor.