This article argues that the extent to which political parties are institutionalized shapes welfare state development. Institutionalized parties allow politicians to overcome co-ordination problems, avoid capture by special interests and form stable linkages with broad social groups. These features both enable and incentivize politicians to pursue extensive welfare policies. The study employs measures of party institutionalization and welfare state features to study these proposed relationships. Even when accounting for country- and year-fixed effects and plausible confounders such as electoral system, unionization, regime type and state capacity, the authors find clear relationships between party institutionalization and more extensive and universal welfare states. Focusing on universalism, they find that the relationship is more pronounced when constraints on executives are strong and in democracies, but that it also exists in autocracies. Further, when disaggregating party institutionalization and evaluating mechanisms, the linkages that institutionalized parties form with social groups constitute one important, but not the only relevant, factor.