As Bernie Sanders thundered and gesticulated this election season, many paid attention. Few had been exposed to such an unflinching social-democratic program, or heard an analysis of society that blamed inequality and exploitation on the “billionaire class.”

But if Sanders inspired allegiance unlike any other left-wing candidate in decades, his decision to run within the Democratic Party invited caution from many on the socialist left. The Democrats, after all, are dominated by representatives of the capitalist class, imposing grave limitations on any left-wing candidacy under its umbrella.

Why was the Sanders wave such a novelty in American politics? And why was the senator forced into such a predicament — choosing between his cherished status as a political independent and any chance of national relevance in a two-party system? The answer lies in a missing labor party. The US is alone among advanced capitalist countries in lacking one. But another question then arises: why is the US such an exceptional outlier?