TEA PARTY IN THE HOUSE: A HIERARCHICAL IDEAL POINT TOPIC MODEL AND ITS APPLICATION TO REPUBLICAN LEGISLATORS IN THE 112TH CONGRESS

We introduce the Hierarchical Ideal Point Topic Model, which provides a rich picture of policy issues, framing, and voting behavior using a joint model of votes, bill text, and the language that legislators use when debating bills. We use this model to look at the relationship between Tea Party Republicans and “establishment” Republicans in the U.S. House of Representatives during the 112th Congress. 1 Capturing Political Polarization Ideal-point models are one of the most widely used tools in contemporary political science research (Poole and Rosenthal, 2007). These models estimate political preferences for legislators, known as their ideal points, from binary data such as legislative votes. Popular formulations analyze legislators’ votes and place them on a one-dimensional scale, most often interpreted as an ideological spectrum from liberal to conservative. Moving beyond a single dimension is attractive, however, since people may lean differently based on policy issues; for example, the conservative movement in the U.S. includes fiscal conservatives who are relatively liberal on social issues, and vice versa. In multi-dimensional ideal point models, therefore, the ideal point of each legislator is no longer characterized by a single number, but by a multi-dimensional vector. With that move comes a new challenge, though: the additional dimensions are often difficult to interpret. To mitigate this problem, recent research has introduced methods that estimate multi-dimensional ideal points using both voting data and the texts of the bills being voted on, e.g., using topic models and associating each dimension of the ideal point space with a topic. The words most strongly associated with the topic can sometimes provide a readable description of its corresponding dimension. In this paper, we develop this idea further by introducing HIPTM, the Hierarchical Ideal Point Topic Model, to estimate multi-dimensional ideal points for legislators in the U.S. Congress. HIPTM differs from previous models in three ways. First, HIPTM uses not only votes and associated bill text, but also the language of the legislators themselves; this allows predictions of ideal points from politicians’ writing alone. Second, HIPTM improves the interpretability of ideal-point dimensions by incorporating data from the Congressional Bills Project (Adler and Wilkerson, 2015), in which bills are labeled with major topics from the Policy Agendas Project Topic Codebook.1 And third, HIPTM discovers a hierarchy of topics, allowing us to analyze both agenda issues and issue-specific frames that legislators use on the congressional floor, following Nguyen et al. (2013) in modeling framing as second-level agenda setting (McCombs, 2005). Using this new model, we focus on Republican legislators during the 112th U.S. Congress, from January 2011 until January 2013. This is a particularly interesting session of Congress for political scientists, because of the rise of the Tea Party, a decentralized political movement with populist, libertarian, and conservative elements. Although united with “establishment” Republicans against Democrats in the 2010 midterm elections, leading to massive Democratic defeats, the Tea Party was—and still is—wrestling with establishment Republicans for control of the Republican party. The Tea Party is a new and complex phenomenon for political scientists; as Carmines and D’Amico (2015) observe: “Conventional views of ideology as a single-dimensional, left-right spectrum experience great difficulty in understanding or explaining the Tea Party.” Our model identifies legislators who have low (or high) levels of “Tea Partiness” but are (or are not) members of the Tea Party Caucus, and providing insights into the nahttp://www.policyagendas.org/