This article takes a novel approach to the question of how bicameralism matters by asking not how it shapes policy outcomes, but rather how it shapes political parties. Bicameralism uniquely challenges political parties because party leaders have few tools for disciplining copartisans in separate legislative chambers. As long as party members do not share identical policy preferences, or strategic contexts across chambers differ, copartisan cohorts in each chamber are likely to favor distinct policy positions. To the extent that parties value clear, consistent party labels, it is in their best interests to find ways to keep intraparty disagreements out of the public eye. We argue that parties in bicameral systems do this by centralizing candidate selection, so that members in both chambers are accountable to the same master. We test our argument using data from 66 political parties in 11 advanced parliamentary democracies.