The federal preemption of state law is a subject that only dorks could love. Four of them (Jon Klick, Mike Petrino, J.P. Sevilla, yours truly) have just published an empirical analysis of preemption decisions in the Rehnquist and the Roberts Courts. Preemption is the Supreme Court’s daily diet, with three or four cases each Term. So you can actually do the numbers.
What the numbers show is that the once-humdrum preemption issue has become a matter of intense ideological contestation. Preemption cases are less likely to be (nearly) unanimous than the general run of decided cases; and in contested cases, the justices tend to break along the usual ideological lines. The four liberal justices are hard votes against preemption except when it comes to preempting state statutes having to do with undocumented aliens. (In those cases, let’s hear it for the feds.) The conservative bloc is much softer: it’s quite possible to gain Justice Thomas’s or Justice Scalia’s vote against federal preemption. But the decisive vote, in preemption cases as in matters involving God, guns, and gays, is Justice Kennedy’s. On the Roberts Court, he has been in the majority in all contested preemption cases but one.
Preemption questions may become yet more controversial down the road. Democratic administrations can’t regulate the economy into the ground all on their own, so they’re happy to make “federalism” noises and unleash state regulators and plaintiffs’ lawyers. A Republican administration would presumably try to block exotic state experiments. That would make for a fun fight.