After being yelled at for months and months for her reluctance to impeach President Trump, Speaker of the House Nancy Pelosi (D-Calif.) abruptly reversed course Tuesday and announced she was in favor of opening an impeachment inquiry against him. The reason is the Ukraine story: "The actions taken to date by the president have seriously violated the Constitution," she said. He "must be held accountable — no one is above the law." Apparently the reports of Trump trying to blackmail Ukrainian President Volodymyr Zelensky during a phone call into investigating his top political opponent, former Vice President Joe Biden, was finally a bridge too far, even for moderate Democrats. At time of writing, a little over 200 House Democrats are supporting the impeachment inquiry, with more joining by the hour.

There are real questions about how serious Pelosi is about this. So far the House has not created a special impeachment committee, or indeed voted on anything at all. But already there are strong signs that impeachment is developing a momentum of its own — something even Pelosi will have trouble slowing if she changes her mind again.

How did the House Democratic caucus shift its thinking on impeachment so quickly? Ryan Grim of The Intercept has a darkly amusing report examining the pivot. Since the 2018 midterm victory, he explains, the leadership's priorities have been twofold. On the one hand, they instinctively coddled a tiny handful of moderate "frontliners" in swing districts, many of whom were ex-military or ex-intelligence cutouts who treat Pelosi and left-wing caucus members with blistering contempt. (People often forget that the "Squad" of leftist women actually supported Pelosi for speaker against a challenge from centrist white men.)

On the other hand, key committee chairs have been eager to make some deals with Trump, and therefore hesitant to truly go after him. Ways and Means Committee Chair Richard Neal (D-Mass.) delayed getting Trump's tax returns so he could get through a retirement bill that would make it easier for shady 401(k) sponsors to rip off employees. Transportation and Infrastructure Chair Peter DeFazio (D-Ore.), who has direct oversight authority over the Trump International Hotel in D.C. (because it is owned by the government), has dragged his feet on investigating the hotel's many payments from foreign governments so he can try to get an infrastructure bill passed.

On top of this, impeachment apparently didn't poll that well in swing districts, and so leadership and frontliners demanded the rest of the party sit down and shut up so they could savvily do nothing until the rising seas finally close over our grateful heads. (Of course such arguments studiously ignored the most relevant historical example — the consolidation of a pro-impeachment majority during Watergate, which took months of investigation.) All told, the Democratic elite was basically fine to twiddle their thumbs while President Caligula made his horse secretary of defense.

But the passivity in the face of Trump's awesome corruption and lawbreaking was creating its own political problems. Base voters have lambasted hesitant Democrats at town halls, and others were giving up on the party. What is the point of voting for Democrats, they asked, if they won't take a single solitary risk to preserve constitutional government? Meanwhile, dozens of scaredy-cat Dems drew primary challenges.

The Ukraine news broke through the deadlock for a variety of reasons. First, it was simple to explain: The president reportedly used taxpayer money to blackmail a foreign government into digging up dirt on a political opponent. "It makes the brazenness of the conduct and the simplicity of the misconduct easy for everybody to understand," Rep. Peter Welch (D-Vt.) told The New York Times. Second, it involved national security. The Emoluments Clause case for impeaching Trump is every bit as strong as the Ukraine scandal, but moderate former troops tend to disregard things that don't involve the military somehow. Finally, it was just one more thing that tipped the scales in favor of the impeachment argument.

And behold! The politics of impeachment are not an automatic disaster for the Chicken Littles in the Democratic caucus. On the contrary, moderate Republican senators like Cory Gardner of Colorado are avoiding reporters asking questions about the topic. Trump himself is manifestly on the back foot. He was extremely triggered on Tuesday night, tweeting all evening and morning about how he is such a pitiable victim, then calling Pelosi out of the blue to try to work out some deal (which she rejected).

Then on Wednesday morning the White House released an astounding memorandum summarizing the call in question, which proved Trump had indeed asked Zelensky to investigate the Biden family. "I would like you to do us a favor," Trump said. "There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that. So whatever you can do with the attorney general would be great." What's more, the memorandum had several suspicious ellipses that appeared to erase sections of the talk where details were discussed — like, perhaps, the $400 million in military aid that The Washington Post reported Trump threatened to withhold unless Ukraine started looking into Biden.

At any rate, just saying the Democrats are going to start looking into impeachment has already put them more firmly in command of political discussion than at any time since they won in 2018. It's an object lesson that polls and the "savvy," "realistic" opinions of political insiders are not all that matters in politics. Sometimes action — even half-hearted — can create its own momentum. Pelosi and company may have no choice but to actually do what the Constitution says when the president is a rotten, filthy criminal: Impeach him.

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