New York mayor Zohran Mamdani and his allies swept a series of congressional primaries in New York City on Tuesday in a remarkable show of strength for the insurgent left that sent shock waves through the Democratic Party.Mamdani’s candidates toppled a pair of incumbents backed by the city’s political establishment, including influential labour unions and the House Democratic leader. Another candidate backed by the mayor won an open House seat, and a handful of democratic socialist challengers he supported were winning down the ballot.For months, Mamdani threw himself and his energised political organisation into the three marquee congressional contests, campaigning late into the night in the race’s final days and calling the election a referendum on the direction of the party.All the winning candidates share Mamdani’s progressive economic platform, and they each ran campaigns that focused intently on ending US support for Israel, a sign of how far public opinion has shifted on the issue, even in New York.[ ‘Up the Bohs’: New York mayor Zohran Mamdani backs League of Ireland club in video messageOpens in new window ]Late on Tuesday night, the mayor stood beaming at a victory party in Brooklyn, where supporters chanted “Free, free Palestine” and “DSA”. Surrounded by many of the same advisers who led his own successful campaign last year, he declared “a new chapter in our party’s history.”“A year ago, it was not the end of a political movement,” he said. “It was the beginning.”Mamdani’s deep involvement amounted to an audacious gamble for a brand-new mayor trying to lead an already fractious city. He alienated key allies along the way, but the pay-offs were far-reaching.At home, the outcome will now cement him as the unquestioned political kingmaker of the nation’s cultural and financial capital and the Democratic Socialists of America as a formidable force.The results also shook the foundations of the Democratic Party far beyond the five boroughs. When they are certified, Mamdani (34) and his movement will be on track to double the number of socialists in Congress from two to four. The outcome will also force a Democratic Party, already searching for its identity, to reckon with its ascendant, unapologetic left.“It’s seismic,” said Jon Paul Lupo, a Democratic consultant who was a top adviser to the city’s last progressive mayor, Bill de Blasio.Supporters of Claire Valdez celebrate after her win in the Democratic primary in Brooklyn. Photograph: Bing Guan/The New York Times