My dissertation is a book project that traces the influence of campaign donors on the political ideology of legislators. Previous studies have observed increasing polarization of American legislatures, yet investigations into the underlying causes have thus far come up short. Lacking from current explanations is a thorough investigation of how money influences the ideology of legislators. In my dissertation, I develop a theory of how legislators respond to the preferences of those who fund their campaigns. I then test this theory using a variety of data. Using an original dataset of state-level campaign contribution limits I show that lowering contribution limits on individuals leads to less polarized legislators, while limits on PACs lead to more extreme legislators. This relationship exists because contribution records suggest that individual donors are more ideologically motivated than political action committees. At the federal level, I show that while legislators are more ideological than co-partisans in their districts, they are quite similar ideologically to the donor class. I show this using an original survey of campaign donors in the 2012 election. Overall, this dissertation suggests that the influence of money is an important factor in the story of polarization in American politics.

I show that legislators respond to the preferences of those who fund their campaigns. I demonstrate that individual contributors are more ideological than PACs and non-contributing voters. Furthermore, I show that limits on campaign contributions dramatically affect the way in which candidates fund their campaigns, yielding the prediction that limits on contributions also affect legislator's ideology and voting behavior. To test this prediction, I use an original dataset of campaign contribution limits in the states over the last 16 years and find that higher individual contributions lead to more extreme legislators, while higher limits on contributions from PACs yield more ideologically moderate legislators.

Understanding why donors give money is vital for developing accurate hypotheses of how money influences politics, yet we still know little about why donors choose give. In this paper I present theories of why PACs and individuals, the two largest sources of campaign money, contribute to political candidates. PACs are primarily motivated by a desire to gain access to legislators and the legislating process while individuals are primarily motivated by ideological considerations. Additionally, a subset of PACs whose interests divide along partisan lines are interested in both. I test these theories using a variety of data and identification strategies. Using an original survey of donors in the 2012 election cycle, I show that individuals consistently rank ideological concerns as most important when deciding who to contribute to. Furthermore, using contribution records and election results, I show differences between individual and PAC contribution patterns. Finally, using two different within-legislator designs, I show a causal relationship between access, ideology and contributions. Among PACs, becoming an incumbent increases contributions by more than 100 percent. Changes in legislator ideology lead to shifts in individual donor behavior but do not affect PACs. These results provide the most direct and comprehensive test of contributor motivations to date.

How well do legislators represent their constituents? This paper addresses this question by investigating the degree of congruence between the preferences of senators and three different constituencies. I find that senators' preferences are nearly perfectly in line with the preferences of the average donor. Senators from both parties are slightly more ideologically extreme than the average co-partisan in their state. Finally, senators' preferences diverge dramatically from the preference of the average voter in their state. The degree of divergence is nearly as large as if voters were randomly assigned to a senator. To estimate the preferences of these groups I use a large survey of voters and an original survey of campaign contributors in the 2012 election cycle. These results show that in the case of the Senate, there is a great deal of congruence between constituents and senators: these constituents however, happen only to be those who write checks and attend fundraisers.

Many pre-election surveys are unable to identify and contact individuals in the likely electorate. Our new methodology draws a likely electorate using unequal probability sampling of publicly available voter files. In seven elections in three states we match or beat forecasts by other polling agencies in 80% of the races we poll in. Our methodology achieved this performance by improving coverage and eliminating interviewer bias.

Do voters turn out more or less frequently when surrounded by those like them? We geocode over 50 million voter registration records in California, Florida, and North Carolina and estimate the effects of racial and partisan composition of neighborhoods at the census block level. We find that a 10 percentage point increase in the out-group neighborhood proportion leads to an approximately 0.5 to 2 percentage point decrease in turnout probability.

In the last 50 years, the distance between Democrats and Republicans has steadily grown. As this distance between the parties grows so too does the list of suggested causes for partisan polarization. In this essay, we review several of the reasons suggested by scholars for the rise in polarization. We briefly evaluate the plausibility of each suggestion and point to ways in which we might better answer the question of why Congress has become so polarized. Finally, we discuss some of the consequences of polarization.