Over the past two decades, scholars of the U.S. Congress have engaged in a long-running, often heated debate over the role that political parties play in shaping outcomes in the institution. Those who argue for strong “party effects” in the institution point to the numerous rules and procedural prerogatives that give party leaders the ability to decide which issues are brought to the floor and how those issues are considered. By limiting the agenda and often closing off the amending process, leaders, under this school of thought, are able to pull policies away from the median of each chamber and toward the preferred position of the majority party. In contrast, those who argue against party effects point out that rules and procedural prerogatives are set by chamber majorities (at least in the House) and that we should not expect the median member of the House to consent to a set of rules and procedures that would leave him or her worse off.

Scholars in both of these camps rely primarily, though not exclusively, on spatial models of the legislative process, assume that members see each issue through the lens of their ideal point, and use highly aggregated roll call data to support their claims. In The Whips, which was recently announced as the winner of the prestigious Richard F. Fenno