Parties are central agents of democratic representation. The literature assumes that this function is an automatic consequence of social structure and/or a product of incentives derived from electoral competition. However, representation is contingent upon the organizational structure of parties. The connection between a party and an organized constituency is not limited to electoral strategy; it includes an organic connection through permanent formal or informal linkages that bind party programmatic positions to social groups’ preferences, regardless of the electoral returns. This article analyzes how the Movimiento al Socialismo (Movement toward Socialism, MAS) in Bolivia and the Frente Amplio (Broad Front, FA) in Uruguay developed two different forms of relationship with social organizations that result from the interplay of historical factors traceable to the parties’ formative phases and party organizational attributes. Party organizational features that grant voice to grassroots activists serve as crucial mechanisms for bottom-up incorporation of societal interests and demands.