More momentum built yesterday among Democrats for impeachment proceedings than on any other single day of the Trump presidency.

Why it matters: One summer phone call by President Trump is proving to be more of an impeachment catalyst for House Democrats than two years of drip-drip revelations from Robert Mueller's investigation. Today, the behind-the-scenes action could burst into view.

"The horse is out of the barn," tweeted Geoff Garin, a pollster for House Democrats. "Saddle up."

What's happening: Pelosi meets today with her six committee chairs leading different strands of the Trump investigation.

Rep. Rosa DeLauro (D-Conn.), a 28-year House veteran and Pelosi ally, issued a statement yesterday calling Trump's phone call with the Ukrainian president "a new chapter in Trump’s egregious conduct," "a reckless abuse of power" and "a turning point."

Last night, seven freshman Democrats — all with military and national security backgrounds — published a Washington Post op-ed saying it will be "an impeachable offense" if, as alleged about the Ukraine phone call, Trump "used his position to pressure a foreign country into investigating a political opponent, and ... sought to use U.S. taxpayer dollars as leverage."

Pelosi, who has tried to tamp down impeachment fever, talked privately yesterday with lawmakers and allies about where they are on impeachment in light of the Ukraine revelations, to gauge whether there is a broad shift within the caucus, sources tell Axios.

"The Speaker is a numbers girl," a Democratic leadership aide said. "The public sentiment has to be there."

Between the lines: You could see the dam breaking Sunday, when House Intelligence Chairman Adam Schiff took a newly aggressive stance on impeachment — comments he made, Axios reported, after consulting Pelosi.

One of the main drivers of Pelosi’s reluctance to embrace impeachment was to protect moderate freshmen in swing districts who won her the majority in 2018. But some of those vulnerable Dems joined last night's op-ed.

The backdrop: All of this is unfolding while Trump is in New York meeting with foreign leaders at the UN General Assembly.

The president has complained in the past when controversies distract from the narrative he wants when he is with other heads of state.

Why Ukraine is different: The Mueller investigation played out through press reports and occasional indictments across two years — softening the blow from the most damning revelations.

The Ukraine story has unfolded in less than a week.

What to watch: House Democrats meet today, and members will be swarmed by reporters. On Thursday, Joseph Maguire, the acting director of national intelligence, will testify in open session before the House Intelligence Committee.

Maguire will be asked about an intel whistleblower's complaint that may have been triggered by the Ukraine conversation. His answers — or even non-answers — could be impeachment bait for Democrats.

Later this week, Trump is to meet at the UN with Ukrainian President Volodymyr Zelensky, who was on the other end of the phone call.

Go deeper: Trump's defiance on Ukraine