Democratic City Councilwoman Helen Gym on Monday endorsed Working Families Party candidate Kendra Brooks’ campaign to join her as an at-large member of Council, fueling a debate among Philadelphia Democrats about whether to support progressives running for office without the party’s blessing.

Brooks, a community organizer in Nicetown who has worked with Gym on education issues, is one of several progressive third-party candidates seeking one of the two at-large Council seats that are reserved for candidates outside the dominant party and have long been held by Republicans.

“For too long, we’ve allowed minority seats on City Council to go to the party of Trump. That’s over,” Gym said in a statement. “Kendra and I have been organizing, protesting, and dreaming together for years. In the face of mass public school closures, she brought her community together and saved her neighborhood school from privatization.”

The debate over whether to shun or embrace the Working Families Party’s campaign has become a proxy war in a broader struggle between the progressive movement in Philadelphia and the city’s Democratic establishment.

“We need to be wary of tradition for tradition’s sake, and rules and practices that are based on these traditions that have been unchallenged and have not been discussed in the modern era,” said State Rep. Chris Rabb (D., Phila.), who also is endorsing Brooks and her Working Families Party running mate, Nicolas O’Rourke. “We have to use the system that we have to maximize our opportunities ... to create stronger alliances between Democrats and others to push a progressive agenda for the benefit of the city.”

Although other liberal Democrats have endorsed Brooks, Gym is the first Council member to do so. The move raised eyebrows in City Hall because it means Gym is effectively encouraging her supporters to forgo voting for one of her four Democratic colleagues vying for at-large seats.

Each voter can only cast ballots for five at-large Council seats in the Nov. 5 election, and each major party can nominate up to five candidates, meaning Democratic voters wishing to support a third-party candidate must skip at least one member of their own party.

Former U.S. Rep. Bob Brady, who chairs the Democratic City Committee, said he received phone calls Monday from ward leaders angry that Gym had endorsed a candidate outside the party. The move could alienate her from Democrats, he said.

“That’s stupid and she shouldn’t do it, and I think it’s going to hurt her in the long run,” Brady said. “They’re taking votes from a Democratic candidate and that’s our slate, and anybody that’s taking votes from a Democrat is the opposition.”

Gym, the first at-large Council candidate in 32 years to win more than 100,000 votes in a Democratic primary, has cultivated a progressive political brand apart from the party.

Brady half-jokingly suggested the remedy might be to remove Gym’s name from the party’s sample ballot, the list of endorsed candidates handed out at polling places on Election Day, and replace it with Brooks’ name. “It’s something we can do if we want,” he said.

Brady also said that he would hold a City Committee meeting to discuss Rabb’s endorsement because he leads the party’s 9th Ward, and ward leaders are prohibited from backing non-Democrats. Rabb said that he made the endorsements in his individual capacity, not on behalf of the ward, and that he had been trying to reach Brady.

Gym declined to respond to questions Monday, citing a family emergency.

Although Brooks and O’Rourke are targeting independents as well, they are unlikely to win without persuading thousands of traditional Democratic voters to join them.

Brooks said that her campaign won’t cost Democrats any seats because the party enjoys such a significant voter registration advantage that an insurgent campaign is unlikely to eclipse a Democrat’s vote total. At-large Democrats typically receive more than 100,000 votes, whereas successful Republican candidates usually take about 35,000 votes.

“My fight is with the Republicans. I’m running against the Republicans who have had a seat at the table and have refused to represent working people,” she said.

Brooks said her endorsements from Gym, Rabb, and Democratic State Reps. Elizabeth Fiedler and Malcolm Kenyatta were “a huge win for the campaign.”

Although Brooks has picked up momentum in recent weeks, most observers still regard the campaigns by the Working Families Party candidates and by Sherrie Cohen, a former Democrat running as a third-party candidate under the “A Better Council” banner, as long shots.

Philadelphia Republicans, however, aren’t ignoring the threat. The progressive outsiders were a topic of conversation at the party’s annual fundraiser, the 35th annual Billy Meehan Clambake, on Sunday in Northeast Philadelphia. And Michael Meehan, chairman of the Republican City Committee, has filed a legal challenge to Cohen’s candidacy. (A judge ordered her removed from the ballot last week, but Cohen is appealing.)

“To me, it’s problematic, and I think it’s an attack on the two-party system,” Meehan said Monday.

None of Gym’s Democratic colleagues have indicated they will follow her lead. Allan Domb, who holds an at-large seat, said he wasn’t aware that Gym would be backing someone outside the party until reached by The Inquirer on Monday. Asked for comment, Domb only said that he would back the five Democrats on the ballot.

“Everyone is entitled to their opinion,” he said, “and I’ll be voting for Helen, myself," and Derek Green, Kathy Gilmore Richardson, and Isaiah Thomas.

Staff writer Chris Brennan contributed to this article.