American Political Science Review

Research Article

Parties, Coalitions, and the Internal Organization of Legislatures


a1 Northwestern University

a2 University of Maryland


We present a theory of parties-in-legislatures that can generate partisan policy outcomes despite the absence of any party-imposed voting discipline. Legislators choose all procedures and policies through majority-rule bargaining and cannot commit to vote against their preferences on either. Yet, off-median policy bias occurs in equilibrium because a majority of legislators with correlated preferences has policy-driven incentives to adopt partisan agenda-setting rules—as a consequence, bills reach the floor disproportionately from one side of the ideological spectrum. The model recovers, as special cases, the claims of both partisan and nonpartisan theories in the ongoing debate over the nature of party influence in the U.S. Congress. We show that (1) party influence increases in polarization, and (2) the legislative median controls policy making only when there are no bargaining frictions and no polarization. We discuss the implications of our findings for the theoretical and empirical study of legislatures.


c1 Daniel Diermeier is IBM Professor of Regulation and Competitive Practice, Kellogg School of Management, Northwestern University, and Director of the Ford Center for Global Citizenship, 2001 Sheridan Road, Evanston, IL 60208 (

c2 Razvan Vlaicu is Assistant Professor, Department of Economics and Affiliate, Department of Government and Politics, University of Maryland, 3105 Tydings Hall, College Park, MD 20742 (


We thank the co-editor and three anonymous referees, Allan Drazen, Tim Feddersen, Matias Iaryczower, Keith Krehbiel, John Morgan, Carlo Prato, David Primo, Allan Wiseman, and Asher Wolinsky for helpful conversations and for comments from seminar participants at the Canadian Institute for Advanced Research (CIFAR), Midwest Political Science Association National Conference 2008, Econometric Society North American Summer Meeting 2008, Wallis Institute Annual Conference 2008, Columbia University, Northwestern University, and the University of Maryland. Financial support from CIFAR and the Ford Center for Global Citizenship at the Kellogg School of Management, Northwestern University, is gratefully acknowledged.