Roll call voting in the U.S. Senate on nominees to the Supreme Court presents political scientists with an empirical puzzle and a theoretical challenge. The empirical puzzle stems from a curious pattern in the nomination politics of recent decades. In some cases, as shown in Table 34.1, the Senate routinely confirms the nominee. In these cases, liberal senators vote for conservative nominees and conservative senators vote for liberal nominees. For example, the most liberal members of the Senate recently voted to confirm judicial conservative Antonin Scalia. But on other occasions – including 9 of the 20 post-Brown-v.-Board of Education confirmations – the confirmation becomes extremely contentious. In these cases many or even most senators vote against the nominee, and voting becomes ideologically polarized. The rejection of Robert Bork illustrates this case.

We therefore face some puzzling questions: Why are some votes consensual? Why are some votes contentious? And what determines voting decisions in both cases? Satisfactory answers to these questions must explain the apparent switching process between the consensual and conflictual votes and the variance within the conflictual votes.