From the abstract for Neal Devins & Lawrence Baum, Split Definitive: How Party Polarization Turned the Supreme Court into a Partisan Court, Supreme Court Review vol. 2016:
Starting in 2010 the Supreme Court has divided into two partisan ideological blocs; all the Court’s Democratic appointees are liberal and all its Republicans are conservative. Correspondingly, since 1990 there has been a dramatic increase in the ideological gap between Democratic and Republican appointees. In this article we make use of original empirical research to establish that this partisan division is unprecedented in the Court’s history, and we undertake a systematic analysis of how it came about. We show that it is linked to growing partisan polarization among political elites, polarization that has shaped the Court in multiple ways. Presidents — for the first time ever — make ideology the dominant factor in appointing Justices. The Senate confirmation process too pays increasing attention to ideology, including party line votes that block the consideration of judicial nominees. Equally significant, the sorting of elites into the two parties on the basis of ideology has greatly reduced the numbers of conservative Democrats and liberal Republicans who might be selected as Justices.
Finally, political elites that tended to lean in a moderate-to-liberal direction during the 1960s through 1980s have become polarized along ideological lines. As we show through original research on the voting patterns of Justices, Justices who once might have been drawn toward moderation are increasingly reinforced in their liberal or conservative orientations. One key reason is that the rise of the conservative legal network has worked against the “drift” of Republican appointees toward more liberal positions that was common a few decades ago. This analysis indicates that the current partisan division on the Court is not a temporary phenomenon; rather, it is likely to continue as long as the current partisan polarization continues.