The outcome of a special Democratic primary to fill a US House of Representatives seat representing northern New Jersey was on a knife-edge on Friday, after a progressive challenger took a surprise lead over a former Democratic congressman who initially appeared to have won the nomination.

The election held on Thursday in New Jersey’s 11th congressional district was prompted by Democrat Mikie Sherrill’s resignation last year after she was elected governor. Eleven Democrats vied to replace her, and on Thursday evening, Tom Malinowski, who represented a neighboring district in the House from 2019 through 2022, took an early lead.

The Democratic National Committee chair, Ken Martin, quickly congratulated Malinowski, but as the ballot count continued, Analilia Mejia, a progressive activist and former political director for Bernie Sanders, overtook the former congressman.

The Associated Press has not yet called the race, but Mejia was leading Malinowski by 488 votes on Friday morning, with more than 61,000 ballots counted.

Mejia avoided calling herself the winner, instead telling a press conference: “I do think that we have emerged victorious, but I want to first make sure that every voter’s voice is heard.”

A victory by Mejia in a state where party leadership has traditionally favored institutional figures could give a boost to Democrats’ progressive wing ahead of the November midterm elections, in which the party hopes to win back control of the House and Senate.

Reacting to Mejia’s momentum, the Texas congressman Greg Casar, who leads the Congressional Progressive caucus, said: “Over and over, voters are sending Democrats a message: we need fighters who will go to the mat for working people.”

The former leader of the New Jersey Working Families Alliance, Mejia entered the contest as a progressive challenger and benefited from high-profile endorsements from Sanders and other leftwing lawmakers including Elizabeth Warren and Alexandria Ocasio-Cortez.

Her campaign emphasized grassroots organizing, economic justice and a rejection of what she framed as establishment politics.

When Mejia campaigned in January alongside Sanders at William Paterson University in New Jersey, she said: “In a moment of rising authoritarianism, of economic insecurity, of state-sanctioned violence, any old blue just won’t do.

“If you send weak sauce to Congress, we will get weak sauce back,” she added.

Malinowski picked up an endorsement from one of the state’s two Democratic senators, Andy Kim, but faced what Federal Election Commission filings say was more than $2.3m of negative advertisements paid for by the United Democracy Project, a Super Pac linked to the American Israel Public Affairs Committee (Aipac).

Patrick Dorton, a spokesperson for the Super Pac, told the New York Times that the group opposed Malinowski because he has recently spoken in favor of conditioning aid to Israel. “That’s not a pro-Israel position,” he said.

In an interview with the Hill, Malinowski, a former diplomat and human rights advocate who served in the Clinton and Obama administrations, defended his stance, saying: “What Aipac seems to be demanding is just blind, blank check support for the current [Israeli] prime minister.

“I don’t think that’s what most pro-Israel Jewish Americans want. I don’t think it’s [what] most Israelis want,” he said.

The winner of the Democratic primary will advance to face the Randolph Township mayor, Joe Hathaway, the sole Republican running in the state’s special general election on 16 April.