Congress by its very nature is a deliberative institution created to mediate societal conflicts and address the policy concerns posed by such conflicts (Madison, Federalist # 10; Dahl, 1967; Cooper, 1970). The challenge for Congress is to embrace these responsibilities without becoming so overwhelmed with internal institutional conflict that its policy processes break down or so regularized and insulated in its policy processes that it fails to see and confront pressing social concerns. Thus a central congressional dilemma: Because conflict is inherent to Congress and threatening to its operation, it is tempted to avoid institutional meltdown by embracing highly constrained and regularized policy procedures. Yet an embrace of excessive constraints can isolate the Congress, inhibit conflict mediation, and allow policy problems to fester. How then does Congress generate the landmark laws that address its central mediational and policy-making tasks (Mayhew, 1991; Binder 2003)? The thesis of this paper is that the capacity of Congress to enact landmark legislation depends significantly on the character and regulation of conflict within the institution. Substantial and sustained landmark productivity requires a Congress that fosters real policy contestation (Dahl, 1967, 1971) characterized by serious conflict and even occasional incivilities, so that difficult policy problems can be brought to its attention (Jones and Baugartner, 2005; Schattsneider, 1960). Such contestation limits the isolation of Congress and connects it with social reality. But Congress then must maintain internal conflict within moderated parameters that avoid institutional meltdown and enable deliberative policy-making to proceed (Cooper, 1970, Part IV; Maass, 1983; Bessette, 1994). In this formulation, too much institutional conflict can inhibit landmark productivity -- but so can too little conflict (Simmel, 1955/1908). Too much conflict, we argue, will occur in polarized Congresses when high party polarization interacts with high inter-party incivility. Too little conflict is witnessed in depolarized Congresses (those below the historic mean level of party polarization) when low party polarization interacts with excessive intra-party civility. Both settings inhibit landmark productivity. In contrast, moderate levels of interactive conflict between party polarization and member incivility foster landmark productivity in both depolarized and polarized Congresses. Institutional conflict thus has countervailing effects, increasing gridlock in polarized Congresses and decreasing it in depolarized ones. To explore the explanatory value of our 'conflict theory' of landmark productivity we examine the statistical relationship between institutional conflict and landmark legislation by Congress from 1891 to 1994. This period begins with the first Congress to occur after the initial passage of the Reed Rules in the House of Representatives and ends with the 103rd Congress, which is the last Congress for which we have complete data. In our analysis: a.We use DW-Nominate scores developed by Poole and Rosenthal (1997) to measure party polarization. b.We determine the rise and fall of incivility within Congress according to the percentage of articles published by the New Your Times and the Washington Post on the Congress between 1891 and 1994 that discuss incidents of congressional incivility... ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]