An important strain of federalism scholarship locates the primary value of federalism in how it carves up the political landscape, allowing groups that are out of power at the national level to flourish—and, significantly, to govern—in the states. On that account, partisanship, rather than a commitment to state authority as such, motivates state actors to act as checks on federal power. Our study examines partisan motivation in one area where state actors can, and do, advocate on behalf of state power: the Supreme Court. We compiled data on state amicus filings in Supreme Court cases from the 1979–2013 Terms and linked it up with data on the partisanship of state attorneys general (AGs). Focusing only on merits-stage briefs, we looked at each AG’s partisan affiliation and the partisanship of the AGs who either joined, or explicitly opposed, her briefs. If partisanship drives amicus activity, then we should see a strong negative relationship between the partisanship of AGs opposing each other and a strong positive relationship between those who cosign briefs.
What we found was somewhat surprising. States agreed far more often than they disagreed, and—until recently—most multistate briefs represented bipartisan, not partisan, coalitions of AGs. Indeed, for the first twenty years of our study, the cosigners of these briefs were generally indistinguishable from a random sampling of AGs then in office. The picture changes after 2000, when the coalitions of cosigners become decidedly more partisan, particularly among Republican AGs. The partisanship picture is also different for the 6% of cases in which different states square off in opposing briefs. In those cases, AGs do tend to join together in partisan clusters. Here, too, the appearance of partisanship becomes stronger after the mid-1990s.