Members of Congress have at times attempted to replace their incumbent party leaders. House minority leaders Joe Martin (R-MA) and Charles Halleck (R-IN), and Speakers John McCormack (D-MA) and Newt Gingrich (R-GA), were all recent recipients of such intraparty challenges, with varying degrees of success. Though such challenges are relatively rare, they can have considerable effects on legislative outcomes: even if unsuccessful, they may alter an incumbent leader’s subsequent behavior in response to the perceived preferences of challengers, or accelerate the leader’s retirement from office. At the same time, the considerable risks associated with such challenges raise the question of what would motivate legislators to undertake such drastic action in the first place. Unfortunately, this phenomenon is relatively understudied, and the factors that make such challenges likely are not well understood. In this project I examine several historical cases of such events as evidence for a possible theory of congressional party leadership insurgency. This includes the 1968 campaign against Speaker John McCormack, for which I have collected data from several sources to reconstruct the likely preferences of Democrats. I will examine what factors motivated the principal organizer of the challenge, Rep. Mo Udall (D-AZ), and use regression analysis to estimate what individual-level variables most influenced legislators’ preferences in that challenge. Comparing this campaign to other challenges in the past thirty years of congressional history, I seek to determine when intraparty leadership challenges are more likely, and what factors increase the likelihood that a party member will support such a challenge. I hypothesize that leadership challenges within congressional parties occur in a series of sequential stages. At each stage, the risks of failure increase, but so too does the possibility of altering the behavior of the incumbent leader, if not removing the leader entirely. Ideological preferences, I argue, do not play as significant a role as one would expect in determining when leadership challenges are likely, or why a legislator would support a leadership opposition campaign. Rather, the likelihood that an individual legislator supports a challenge is affected by two factors: the degree to which the leader is perceived to have accomplished the collective goals of the party, and the particular stage of the leadership challenge. Early stages of an opposition campaign are unlikely to weaken individual members’ support for the incumbent, but the momentum of later stages bring with it greater support from partisans. This research has the potential to provide several important contributions to our understanding of congressional party politics and leadership. Identifying motivations for opposing incumbent party leaders may shed light into what legislators see as the most important roles or responsibilities for their leaders. If, as many congressional theorists claim, these leaders are “agents” of their party, this will help us determine the most critical tasks delegated to such “agents” by their caucus. In addition, given the importance of party leaders in shaping and driving the legislative process, knowing when and why they are likely to be replaced or challenged will contribute to our understanding of the legislative process more generally. [ABSTRACT FROM AUTHOR]