Most democracies are governed by coalitions, comprising multiple political parties with conflicting policy positions. The prevalence of these governments poses a significant question: Which parties’ electoral commitments are ultimately reflected in government policy? Recent theories have challenged our understanding of multiparty government, arguing that the relative influence of coalition parties depends crucially on institutional context. Specifically, where institutions allow credible enforcement of bargains, policy should reflect a compromise among all governing parties; where such institutions are absent, the preferences of parties controlling the relevant ministries should prevail. Critically, empirical work has thus far failed to provide direct evidence for this conditional relationship. Analyzing changes in social protection policies in 15 parliamentary democracies, we provide the first systematic evidence that the strength of legislative institutions significantly shapes the relative policy influence of coalition parties. Our findings have implications for our understanding of coalition government, policymaking, and electoral responsiveness.