It has happened before: an upset victory by a Democratic Socialist in an important primary election after an extraordinary grassroots campaign.

In the summer of 1934, Upton Sinclair earned the kind of headlines that greeted Zohran Mamdani’s primary victory on June 24, 2025, in the New York City mayoral election.

Mamdani’s win surprised nearly everyone. Not just because he beat the heavily favored former governor Andrew Cuomo, but because he did so by a large margin. Because he did so with a unique coalition, and because his Muslim identity and membership in the Democratic Socialists of America should have, in conventional political thinking, made victory impossible.

This sounds familiar, at least to historians like me. Upton Sinclair, the famous author and a socialist for most of his life, ran for governor in California in 1934 and won the Democratic primary election with a radical plan that he called End Poverty in California, or EPIC.

The news traveled the globe and set off intense speculation about the future of California, where Sinclair was then expected to win the general election. His primary victory also generated theories about the future of the Democratic Party, where this turn toward radicalism might complicate the policies of the Democratic administration of Franklin D. Roosevelt.

What happened next may concern Mamdani supporters. Business and media elites mounted a campaign of fear that put Sinclair on the defensive. Meanwhile, conservative Democrats defected, and a third candidate split progressive votes.

In the November election, Sinclair lost decisively to incumbent Gov. Frank........

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