What explains mass party formation? Prevailing approaches explain party formation as a process of reflection of preexisting social constituencies, or as the consequence of the rise of the bureaucratic state and in particular the advent of universal suffrage. These approaches fail to explain why Mexico's Partido Revolucionario Institucional (PRI) emerged as a mass party in some regions but not others despite attempts to do so and similarity in conditions that have been posited as central to party formation. I put forth a novel approach that posits that parties emerge as mass organization through a process of constitution of the very social base they claim to represent, but their constitutive powers are conditioned by fundamental economic structures. Relying on agrarian censuses and archival data, I show that the PRI emerged as a mass party in areas where land privatization had been more intensive. In these areas the party during its process of formation was able to build new, and absorb existing, peasant unions and organizations and carry out strong electoral mobilization. These findings suggest that mass party formation is dependent on the destruction of "precapitalist agrarian structures.