For fifty years, Bolivia’s political party system was a surprisingly robust component of an otherwise fragile democracy, withstanding coups, hyperinflation, guerrilla insurgencies, and economic chaos. Why did it suddenly collapse around 2002? This article offers a theoretical lens combining cleavage theory with Schattschneider’s concept of competitive dimensions for an empirical analysis of the structural and ideological characteristics of Bolivia’s party system from 1952 to 2010. Politics shifted from a conventional left-right axis of competition, unsuited to Bolivian society, to an ethnic/rural–cosmopolitan/urban axis closely aligned with its major social cleavage. That shift fatally undermined elite parties and facilitated the rise of structurally and ideologically distinct organizations, as well as a new indigenous political class, that transformed the country’s politics. Decentralization and political liberalization were the triggers that politicized Bolivia’s latent cleavage, sparking revolution from below. The article suggests a folk theorem of identitarian cleavage and outlines a mechanism linking deep social cleavage to sudden political change.