This dissertation examines unequal political representation in the United States. Specifically, I ask whether the political opinions of citizens with low incomes are underrepresented by the policy choices made by their state governments as compared to more affluent citizens. This question, I argue, is central to evaluating the quality of American democracy. Using a wide array of survey data, I first demonstrate that political preferences actually differ across income groups. This is important because if preferences do not vary, then it would be impossible to distinguish whose opinions are better reflected in the public policy decisions made by elected officials. I then systematically evaluate the equality of political representation using three different measurement techniques. Regardless of the particular measure used, citizens with an annual household income of less than $35,000 per year are always the least likely to have their opinions reflected in policy. I also construct an original Political Inequality Index that ranks the states according to the degree to which the opinions of citizens with low incomes are neglected in state policies compared to the opinions of the middle class and the wealthy. I then take advantage of the spatial variation in political inequality across the states to examine possible explanations, an important inquiry that has received little systematic analysis to date. First, I assess political and behavioral explanations and find that representation is more equal in states that have more competitive legislative elections, where the poor turn out to vote at higher rates, and states with a larger interest group community, provided that the community is not dominated by for-profit interests. Second, I assess institutional and legal explanations and find that representation is more equal in states that make it easier to access the ballot initiative process, impose stricter regulations on campaign contributions, and provide more staff and resources to their state legislators. I conclude by discussing how this study relates to the state politics and 'unequal democracy' literatures more broadly and by highlighting a series of concrete policy recommendations that would help promote a more equal weighting of citizens' political preferences in the state policymaking process.