Even with a couple of weeks’ distance, Zohran Mamdani’s election still feels almost unreal, like a dispatch from an alternate universe. But it’s happening: America’s largest city is about to be led by a young socialist, a Jacobin subscriber, and a committed member of Democratic Socialists of America (DSA).

For many of us on the New York left, this is the high point of our political lives thus far, a vindication of patient, often grinding organizing. But amid the celebration, there is a note of worry — the feeling that we have won the sprint, and now the marathon begins. For November’s result to mark the beginning of a lasting transformation of not only one city but national politics, we have to start by acknowledging an uncomfortable truth: despite our impressive electoral reach, our movement’s roots are still shallow.

Things were very different when American socialism had its first big electoral breakthrough. The 1910 election of Socialist mayor Emil Seidel in Milwaukee, Wisconsin, initiated nearly fifty years of leftist governance in the city. Milwaukee’s triumph was the culmination of decades of escalating working-class militancy and socialist growth. Mamdani’s, by contrast, has taken place despite far lower levels of union and left organization. Consider the raw numbers: while Milwaukee’s Socialists had roughly one member for every one hundred city residents, New York’s chapter of DSA has, even after a big surge in the past decade, one member for every 670 residents.