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.