Why and how do class cleavages become politicized in party systems? The recent tenure of left governments in South America has seen the party system realigned along the class cleavage in Venezuela but not in Brazil or Chile. This article argues that different approaches to expanding social protection—"mobilizational" in Venezuela and "technocratic" in Brazil and Chile—explain divergent outcomes. Mobilizational policies incentivized social organization among beneficiaries and allowed for linkages between these organizations and left parties, facilitating credit claiming and recruitment by the left. Technocratic policies targeted atomized beneficiaries with no room for partisan involvement, making credit claiming and recruitment difficult. Case-based evidence illustrates these mechanisms whereas survey analysis using genetic matching confirms that mobilizational and technocratic programs exerted strikingly different effects on support for left parties. The broader contribution is to theorize a new form of class-based political mobilization in Latin America, also being implemented in other countries.