Current theories employ a transactional approach to explain how a given constituency successfully advances its policy agenda. These theories focus on groups’ capacities to marshal material, mobilizational, or disruptive resources to exert influence on politicians. However, these theories cannot account for how constituencies lacking such resources nonetheless succeed at promoting their demands. We analyze two cases that involved the promotion of divisive issues—the decriminalization of abortion in Bolivia and the passage of a law recognizing transgender rights in Uruguay—to explain how social groups can advance their agendas. We show how these groups can engage in constitutive relations with parties, i.e., they influence party decision-making processes by acting as constituencies inside parties. In parties with formal and/or informal structures that allow their members to exercise voice, constituencies can exert agency and transform a party’s policy agenda without engaging in resource transaction. We outline this theory through two in-depth case studies using a systematic process-tracing analysis.