Government-initiated referendums (GIRs) have so far been neglected by the debate on the effects of institutions on policymaking in presidential systems. The literature on Latin American politics has focused on isolated cases of GIRs, which are largely interpreted as epiphenomenal to a regional trend toward personalistic neopopulism. This article provides a conceptual framework for the systematic comparative study of GIRs. It argues that presidents' propensity to promote legal changes through referendums and their concomitant capacity to dominate policymaking are subject to the interaction of two institutional variables (constitutional rules regulating the competences of elected officials in GIR processes and minimum turnout requirements) and two political variables (preference distribution in the legislature and the position of the median voter). These propositions are tested through a comparative analysis of referendum experiences in Colombia and Bolivia, two cases with similar political settings and significant variation in each of the institutional variables.