Do party leaders influence roll-call voting in congress?

Abstract Scholars typically assume that congressional party leaders whip their members and influence their voting behavior, but little evidence convincingly separates the effects of party leaders from the selection of members into parties. I find that a switch from a relatively moderate to a relatively extreme party leader causes rank-and-file members to cast more extreme roll-call votes (and vice versa). I further find that party leaders even influence the members who did not support their leadership bid, and rank-and-file members are less likely to cast partisan votes when there is no party leader. This study also sheds light on a historically anomalous period of Republican moderation, and it helps to explain the increase in congressional polarization over the past 50 years.