The largest online progressive organization in the United States is breaking with House Democratic leadership and endorsing a primary challenge to Rep. Joe Crowley of New York, who is regularly floated as the next potential speaker of the House.

The endorsement of challenger Alexandria Ocasio-Cortez, who’s running to represent the 14th District covering parts of the Bronx and Queens, gives a significant lift to her long-shot campaign in its final days, as Crowley has been blanketing the airwaves with television ads. The endorsement, regardless of its impact on the race, is extraordinarily damaging to Crowley’s hopes for leading the Democratic Party in the House once Rep. Nancy Pelosi of California steps aside.

Crowley has been positioning himself for a run as a liberal alternative to Pelosi’s longtime No. 2, Steny Hoyer of Maryland. But he had done so relying on an outdated frame of what constituted progressive politics, raking in Wall Street cash and generally marrying corporate-friendly politics with liberal approaches to marriage equality, environmental protection, and reproductive freedom. The race against Ocasio-Cortez has also brought an unhelpful spotlight on the inner workings of Crowley’s Queens machine, which runs a foreclosure mill, reaps profits from families who die without wills, and enriches Crowley’s friends and relatives.

The blow from MoveOn comes within a week of another setback from the left. After Crowley solicited an endorsement from Rep. Ro Khanna, a progressive freshman from Silicon Valley, Khanna faced a firestorm of protest. He quickly backtracked, announcing a dual endorsement of both Crowley and Ocasio-Cortez, effectively nullifying his initial nod.