Zohran Mamdani has shocked the political establishment by turning the New York City mayoral race into a nail-biter. How has a thirty-three-year-old Muslim socialist gotten this close to heading the world’s most economically and politically important city? Some pundits have pointed to Zohran’s charisma and brilliant use of social media. Others have stressed the strength of his volunteer canvassing army as well as the popularity of his policies to lower living costs. All those things are true.

But one piece of the puzzle has so far been overlooked: Zohran has received just enough labor support to prevent this race from becoming an Andrew Cuomo cakewalk.

Thousands of rank-and-file supporters have successfully pushed some of the city’s biggest unions to endorse Zohran or at least not endorse Cuomo. Though not all union leaders have been brave enough to buck the political establishment, New York City’s young leftist upstart has generated enough labor support to come within electoral striking distance. The union vote could be decisive in a tight, low-turnout primary election. And union power will be pivotal for overcoming the billionaire-backed onslaught that is sure to come if Zohran wins the June 24 primary.