Uruguay’s Frente Amplio (Broad Front, FA) is a mass-organic institutionalized leftist party. It is a deviant case that helps explicate the reproduction of mass-organic party organization and activism. The peculiar internal structure of the FA and the rules that ensure a role for grassroots activists in the highest decision-making bodies of the party are a product of the extraordinary conditions that existed at the time of the party’s birth in 1971. The intense autonomous activism that occurred during this stage acts as a historical cause. The decision-making authority of the grassroots activists, granted incrementally since the FA’s foundational stage, enables activists to block changes that reduce their power, engendering a lock-in effect and positive feedback. These rules grant FA activists a significant voice, which imbues activists´ participation with a strong sense of efficacy. This perceived efficacy operates as a selective incentive for activists to engage with the party. The chapter develops this theoretical argument and justifies the selection of the FA as a deviant case of party reproduction.