Political parties with activists are in decline due to various external shocks. Societal changes, like the emergence of new technologies of communication have diminished the role and number of activists, while party elites increasingly can make do without grassroots activists. However, recent scholarship concerning different democracies has shown how activism still matters for representation. This book contributes to this literature by analyzing the unique case of the Uruguayan Frente Amplio (FA), the only mass-organic, institutionalized leftist party in Latin America. Using thick description, systematic process tracing, and survey research, this case study highlights the value of an organization-centered approach for understanding parties' role in democracy. Within the FA, organizational rules grant activists a significant voice, which imbues activists' participation with a strong sense of efficacy. This book is an excellent resource for scholars and students of Latin America and comparative politics who are interested in political parties and the challenges confronting new democracies.