It looked like progressive Chris Rabb had a mountain to climb to win the Democratic nomination for a congressional district in Philadelphia.

The mayor and the city’s Democratic Party had endorsed another candidate in this week’s primary. So had members of Pennsylvania’s delegation in the U.S. House. One Rabb rival was backed by millions of dollars. A second benefited from a get-out-the-vote operation run by the influential local building trades unions.

But Rabb finished 15 percentage points ahead of his closest competitor in Tuesday’s election, and the state representative is likely on his way to Washington because no Republican sought the GOP nomination.

Rabb was propelled by a constellation of progressive groups, charting a path to victory partly by assailing his own party as listening more to donors than voters. He credited a grassroots movement inspired by New York City Mayor Zohran Mamdani, alarmed by Republican President Donald Trump and hungry for a government responsive to their needs.

“That was at the heart of why I was running and that is what I built my campaign around and that, I believe, is a chord we struck in this electorate that showed up and came out like gangbusters,” Rabb said in an interview with The Associated Press.

The progressive left counts Rabb’s success as one of its biggest victories of the year and the latest warning sign that Democratic voters see the party’s leadership as weak and feckless in countering Trump. Progressives are also running for House seats in New York, California and Michigan where they are challenging Democratic incumbents or aiming to take on vulnerable Republicans.