We disentangle constituent and partisan influences in Congress by taking advantage of a largely unexamined institutional settinglame-duck sessions. Lame-duck sessions of Congress are comprised of exiting members, who are freed from both constituency and party constraints, and returning members, who face a significantly reduced constituency constraint but a still strong party constraint. Comparing exiting and returning House members thus provides meaningful leverage in assessing the constraining influence of party. In the regularly occurring lame-duck sessions between 1877 and 1933, exiting House members exhibited greater movement away from the median party position than did returning members, consistent with expectations regarding party influence. In addition, party leaders ability to apply pressure was significantly reduced in lame-duck sessions due to the presence of a large group of exiting members. Finally, majority-party leaders were able to exercise negative agenda control in lame-duck sessions when their party maintained control of the next Congress, but they often acted to roll their own party members (an occurrence we dub a strategic roll) when their party lost control of the next Congress, as a way to minimize policy loss. In the post-1933 era, after the passage of the 20th Amendment, lame-duck sessions (those portions of the second session that stretch beyond the November elections) are more accurately characterized as extensions of regular sessions, with party leaders ability to pressure members and exercise negative agenda control remaining virtually constant across sessions. Lower levels of turnover in the modern era appear to contribute to an enhanced ability of majority-party leaders to wield influence.