Party-centered theories of Congress often rely on the critical assumption that some majority party members vote against their preferences when granting their leadership procedural powers, such as closed rules. Such an assumption renders these approaches ad hoc, and thus theoretically dubious, unless firm support for the assumption can be found.

Firm support is elusive largely because it is difficult to separate party and preference effects. In this article, we produce a simple but critical test of the party persuasion assumption that largely avoids these measurement problems. Specifically, we use a “switcher analysis” (Krehbiel 1998) to compare votes on final passage of the legislation with the votes on the closed rule. Our analysis of all closed rule-final passage vote pairs for the 104th–108th Congresses reveals vote patterns that cannot exist absent significant party effects.