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 powerless constituencies succeed at promoting their demands. We analyze two least likely cases of 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 powerless groups can advance their agenda. We show how powerless groups can engage in constitutive relations with parties to influence party decision-making processes by acting as constituencies inside parties. In parties with formal and informal structures that allow their members to exercise voice, powerless constituencies can exert agency and transform a party’s policy agenda. We outline this theory through two in-depth case studies using a systematic process-tracing analysis.