From LSE: Why, then, do Democratic and Republican incumbents diverge so much, ideologically? In my ongoing book project, I argue that we have missed a key factor in the ideological divergence of candidates and, as a result, in the growth of polarization. Most of our explanations for polarization focus on the changing demands of voters; I focus instead on the changing supply of candidates. Building on the citizen-candidate model, I argue that when the costs of running for office are high, and/or the benefits of holding office are low, the supply of candidates will become more ideologically extreme. I employ a variety of analyses that find strong support for this candidate-supply theory. And because, as I argue, the costs of running have gone up (because of increased fundraising burdens, media scrutiny, and more) and the benefits have gone down over the past few decades (because of rising opportunity costs in non-legislative careers, among other factors), the candidate supply has helped lead to the high levels of legislative polarization we see today.
Elections are the key mechanism by which voters control their representatives in a democratic society. To understand how, and to what degree, voters succeed in constraining the behavior of those to whom they delegate power, we need to understand how they go about choosing representatives for office. In my research I shed light on one small part of this much broader process. When voters in closely contested primary elections nominate a more ideologically extreme candidate, the general election strongly penalizes their choice. As a result, the general election is a strong filter on the candidate supply, sending to office those candidates who, on average, are more moderate than their opponents.
But the power of voters to select for ideologically moderate candidates is limited. Voters can only choose from among those people willing to run for office in the first place. Thus, while they may choose to support relatively moderate candidates, in times when the candidate supply is extreme, they will have no choice but to elect a relatively extreme candidate. To understand the root causes of polarization, as well as to understand the electoral process more generally, we need to examine not only the way voters choose among candidates, but the way citizens choose to become candidates.