This story is updated with results below.

Philadelphia has the potential to elect two Working Families Party candidates to its city council on Tuesday, replacing Republicans in two seats the GOP has held since the 1950s. The seats, by the city’s charter, belong to a minority party. If the WFP managed to supplant the GOP, the city council would be dragged dramatically to the left. The Democratic Party is doing everything it can to halt that progress.

Over the last few months, the city’s bosses have threatened to remove party committee members who back the third-party candidates from their posts within the party. (It’s unclear what steps the party would take to confront rank-and-file or at-large officials who don’t hold leadership positions but decided to back non-Democrats. Ward leaders risk losing their titles.)

The drama over the surging campaigns of both Kendra Brooks and Nicolas O’Rourke, two longtime community organizers backed by the WFP, that’s played out over the last several months echoes the ongoing debate national Democrats are having over how to deal with an insurgent progressive wing — except in this case, the object of protection is not incumbent establishment Democrats, but Republicans.

The Democrats’ House campaign arm, the Democratic Congressional Campaign Committee, issued a blanket policy earlier this year cutting off consultants and firms working with primary challenges, which has only seemed to energize progressive candidates and the firms working with them. Philadelphia has taken the same stance toward third-party candidates its Democratic officials want to support, going out of its way to warn committee participants that their positions could be in jeopardy if they back someone who isn’t Democrat. The results of Tuesday’s election could demonstrate whether that strategy is effective, at least at the local level.

In August, Philadelphia Democratic Party Chair Bob Brady, a former U.S. Congressman and the longtime boss of the local party machine, called Democratic Council Member Helen Gym’s decision to endorse Brooks “stupid,” the Philadelphia Inquirer reported. Gym herself was elected as an insurgent in 2015. And in October, the city party posted a warning to its Facebook page that if any committee member backs a city council candidate who isn’t a Democrat, they’ll lose committee membership. Ward leaders are also prohibited from backing non-Democrats. The Montgomery County Democratic Committee put out a similar statement the same day, saying that not supporting Democratic candidates “establishes dangerous precedents and is disrespectful to Democratic voters” and that “the Party is obligated to support duly nominated Democrats against opposing parties in the General Election.”

The twist is that Brooks and O’Rourke are each running for two at-large seats reserved for minority parties — meaning that Democrats aren’t eligible for either one. What it comes down to, in the view of the Democratic establishment, is that voting for independents could siphon votes away from five Democratic candidates running for blue at-large seats. There are 17 seats on the city council, currently held by 14 Democrats and three Republicans. The council’s seven at-large members don’t directly represent a district. Republicans have held two of the at-large seats for decades, but consistently receive only a fraction of the votes council Democrats do. Democrats say they don’t want voters giving one of their five votes for at-large candidates away to third-party candidates because it takes away votes from other Democratic at-large candidates.

But those Democratic seats aren’t really at risk. Even if voters use one of their five votes for a third party candidate over a Democrat, the chances that a Democrat still wins the other five seats is pretty high. Democrats have controlled the council since the current city charter was enacted in 1951.

The WFP says that even in a city as consistently blue as Philadelphia, there hasn’t been a coordinated strategy to knock Republicans off the city council, let alone replace them with true progressives. For its part, the WFP projects that it will spend over $400,000 on Brooks and O’Rourke’s campaigns, and have knocked on 150,000 doors and sent 300,000 text messages.

Both Brooks and O’Rourke see their candidacies as part of a broader push to one day build a progressive majority on the city council, working together with progressive Democrats like Gym to advance priorities including tax reform, affordable housing, tenants’ rights, and continuing to expand rights for retail and service workers. Both candidates are endorsed by progressive groups like the Philadelphia chapter of the Democratic Socialists of America, Unite HERE Philadelphia, Make the Road Action in PA, and Sunrise Movement’s Philadelphia chapter.

They say they’re tapping into a base of supporters who feel disenchanted by the current two-party system and the last several years of machine politics in the city. “I think we’ve already made history, at least here in Philadelphia,” O’Rourke said, “because of the visceral response that we’re seeing from the establishment.”

The shifting demographics among Philadelphia’s registered voters also indicate that independents may be leaching more votes from Republicans than from Democrats, as is commonly thought. The number of registered independent voters in Philadelphia surpassed the number of registered Republicans for the first time this year, reaching 125,693. The number of registered Republicans in the city has decreased by 17,470 over the last 10 years. If anything, a city council with majority Democratic control and two third-party candidates could be a more accurate reflection of the city’s registered voters.