Summer Lee, a progressive Democrat, wins the primary for a House seat in Pennsylvania.

PITTSBURGH — State Representative Summer Lee, a progressive Democrat who could become the first Black woman to represent Pennsylvania in Congress, won an expensive and fiercely fought primary battle on Friday after three days of vote-counting, defeating a more centrist contender who was the favorite of the party establishment.

After a string of primary losses for the national left-wing movement in 2021 and a mixed record in the first months of 2022, Ms. Lee’s narrow victory, called by The Associated Press, amounts to a significant win for that slice of the party, amid a vigorous battle over the direction of the Democratic Party that will be playing out in races around the country over the coming weeks.

Ms. Lee, 34, who overcame heavy outside spending against her, had the endorsements of leading progressive figures including Representative Alexandria Ocasio-Cortez of New York, Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, and local figures including Mayor Ed Gainey of Pittsburgh and some labor groups. Mr. Sanders held a rally with Ms. Lee last week.

Ms. Lee defeated her chief rival, Steve Irwin, a lawyer and former head of the Pennsylvania Securities Commission who had amassed substantial support from the party establishment. Mr. Irwin gained the endorsement of Representative Mike Doyle, whose retirement opened the seat.

In a statement before the race was called, Mr. Irwin called Ms. Lee a “passionate, dynamic voice and strong leader for our region.”

Even before the race was called, left-leaning leaders and organizations in the party were declaring victory while more moderate party strategists seemed demoralized by the result, even as more centrist candidates won other races this week.

“Against an obscene amount of dark money, Summer Lee pulled off a stunning victory,” read a fund-raising appeal from Ms. Ocasio-Cortez’s team. Her “victory demonstrates the strength of the growing, organized progressive and democratic socialist movement,” the message said.

Among the outside groups that intervened in the race was a super PAC aligned with the American Israel Public Affairs Committee, which paid for a barrage of advertising attacking Ms. Lee.

She also defeated candidates including Jerry Dickinson, an associate professor of law at the University of Pittsburgh.

Ms. Lee, who won a 2018 State House primary with the backing of the Democratic Socialists of America, is a supporter of sweeping policies including Medicare for All and the Green New Deal, and she has spoken forcefully about the need to defend abortion rights and combat racial injustice.

The 12th District in the Pittsburgh area is considered safely Democratic in the general election.