In recent decades, partisan polarization has not only grown but also extended to a wide variety of political processes. Despite this fact, we lack a strong understanding of the policy process under conditions of relatively extreme polarization. The roles of interest groups and think tanks in this environment are particularly understudied. How have research producing institutions changed in response to partisan polarization? And which types of organizations are most influential under the current system? Existing theories often lack an appreciation of the role of partisanship and assume relatively unstructured pluralistic competition in the development and debate of public policy. To help rectify this limitation, I view actors in the policy process as part of a system structured mainly by partisan dynamics. This changing “marketplace for ideas” has incentivized some interest groups and think tanks to invest in a single political party to enhance their influence and accomplish their goals. The result is a policy system defined by two competing extended party networks, or loose coalitions of formal party members and outside research institutions. Under such conditions, influence accrues to those organizations that engage in relatively partisan research and advocacy efforts. In the first empirical chapter, I use interview results to show that members of Congress require information, ideas, and talking points from outside actors. These politicians seek out resources that will help further their individual and partisan goals (which are increasingly distant from the opposing party). This decision-making calculus has downstream effects on research institutions, incentivizing many to become party allies across a range of issue areas. Not all groups have responded this way, though. There still exists a cohort of academic and/or politically moderate organizations that have changed more slowly in response to increased polarization. Problematically, groups that are both partisan (in terms of their preferences and behavior) and political (in terms of their direct advocacy strategies) are growing in number and influence. In the second empirical chapter, I levy a wide range of evidence to show that these “Partisan-Political” groups are designed to produce research that consistently supports a particular ideological or partisan vision. These types of groups also develop reputations as key party allies and thick ties to members from their preferred party. In the final empirical chapter, I use a case study of the debate surrounding cap-and-trade regulations to show that these partisan advocacy strategies pay off, as Partisan-Political groups have significant influence over the discourses of their preferred party. In general, the textual and network analyses in this chapter demonstrate that ideas and talking points flow mostly along party lines, prohibiting compromise and allowing extended party actors to institutionalize their preferences. Thus, there is significant evidence for “polarized policymaking”, with two relatively distinct extended party networks developing alternative ideas and discourses in policy debates. These findings have implications for the presence and continuation of partisan polarization, the legislative process, and democratic representation.