The contentiousness of Senate voting on Supreme Court nominations increased dramatically from 1937-2010. We identify four potential sources of the increase: 1) changes in the Senate; 2) changes in the nominees; 3) changes in the political environment; and, 4) changes in senators' evaluative criteria. Using new data and improved statistical techniques, we estimate a well-performing model of senators' individual voting choices on Supreme Court nominees. Simulations allow an evaluation of the contribution of the four classes of factors to increased contentiousness. The principal source of increased contentiousness was the combination of increasingly extreme nominees and an increasingly polarized Senate. Also significant was the increased mobilization of interest groups. In sum, increased contentiousness seems largely to reflect the ideological polarization of American political elites.