Zohran Mamdani ran his campaign for New York City on two messages: making the city affordable and taxing the rich. This has been a winning formula for many progressive candidates for more than a century.

But history also reveals a more sobering lesson: you can’t finance progressive policies with a regressive economy. Social democracy in New York City and elsewhere has repeatedly learned this lesson the hard way. To be fiscally dependent upon the same wealthy individuals and firms who displace working-class residents, contest our policies, and undermine our public finances is profoundly self-defeating.

That’s why past progressives and socialists, from the Knights of Labor to the Wisconsin sewer socialists, didn’t just look to tax wealthy individuals and firms — they looked to diversify urban economies so they wouldn’t depend as much on the wealthy to begin with. By cultivating public enterprises and worker-owned firms, and by aspiring to build economies organized around the needs of working-class residents, these radicals tried to create cities that delivered affordability and justice. They recognized that letting the private economy produce ever more inequality, then trying to tax those at the top to redistribute sufficiently to everyone else to correct structural imbalances, was an impossible task.