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This paper presents a statistical study of the published preemption decisions of the United States Courts of Appeals over the past five years. It tells a story consisting of two distinct parts. The first part is that preemption disputes seem to produce a large measure of judicial consensus. In the full universe of cases, there is only a slight difference between Republican and Democratic appointees: Republican judges voted for outcomes favoring the preemption of state law at a rate exceeding Democratic judges by a margin of roughly 5 percent. This difference is not trivial, but it is not statistically significant at a 95 percent level of confidence. Moreover, this overall similarity was not the product of Republicans and Democrats disagreeing in different clusters of cases, such that their aggregate voting records masked fairly frequent clashes. Rather, more than 94 percent of the circuit courts’ published preemption decisions were unanimous, a rate that exceeds our best estimates of the mean for published Court of Appeals decisions as a whole. The second part of the story is that, despite this general consensus, there remain some important differences between Republican and Democratic appointees. When the judicial panel was ideologically homogenous (either all-Republican or all-Democrat), Republicans were substantially more likely than Democrats to vote in favor of preemption. Moreover, in the most contested preemption cases - those in which at least one Republican and one Democrat served on the panel, and at least one judge dissented - Republican appointees were more than three times as likely as Democratic appointees to vote in favor of preemption (roughly 72 percent versus 22 percent). Thus, the full picture of preemption decisions in the Courts of Appeals is intriguing, if somewhat unsurprising. In general, preemption cases do not provoke much ideological fissure among federal circuit judges. But in particular circumstances - especially the small subset of preemption cases where judges disagree on the result - party affiliation appears to be highly predictive of how that disagreement will play out. In these marginal cases - where the accepted sources of legal authority fail to control the outcome, and the norms of consensus and collegiality are insufficient to restrain judicial dissent - the pattern is unmistakable: Republicans are far more likely than Democrats to favor the federal preemption of state law.



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