In this article, we develop an institutional politics theory of public social provision and examine U.S. social spending programs at the end of the New Deal. This theory integrates key insights of institutional and political theories of social policy. Drawing on institutional arguments, our theory holds that the willingness or ability of pro-spending actors to promote social spending initiatives depends on institutional conditions, especially the extent of voting tights and the nature of political party systems. Furthermore, drawing on political arguments, the theory posits the importance of pro-spending actors, including progressive factions of political parties and organized challengers. To appraise the institutional politics theory, we analyze state-level outcomes for Old-Age Assistance pensions and Works Progress Administration wages, employing multiple regression and qualitative comparative analysis (QCA). All analyses support the institutional politics theory.