Fifty years after its last high-water mark in American life, the dubious doctrine of interposition is back in vogue. Once offered up in vain by desperate southern leaders as a legal justification for blocking federal civil rights initiatives, including Supreme Court rulings, it is now being used by opponents of health care reform as they marshal their forces with an eye toward post-passage litigation challenging the constitutionality of the new legislation.
The largely discredited doctrine posits that the effect of Supreme Court decisions or other "encroachment" by the federal government may validly be blocked by the "interposition" of a viable state right (say, under the 10th Amendment) between the feds and the subject of the consitututional pronouncement. It's the theoretical equivalent, in other words, of a state official standing in front of a schoolhouse door blocking a lawfully-registered student from entering. It garners a lot of heat, and even a little bit of historical light, but has no tangible support in modern American law.
The Supreme Court has never, ever ruled the doctrine valid or permissible as a defense to the enforcement of a federal law. In fact, even the most strident states-rights advocates among the Justices have drawn the line at the implementation of interposition, if not for legal reasons than surely for practical ones. No surprise, of course, because interposition would put to a popular vote, state to state, the question of whether to follow controversial Supreme Court decisions and orders. In so doing, it would directly undercut the Court's preeminent ruling, in Marbury v. Madison, which forms the basis for the way constitutional law is made; with final judicial review resting solely with the Justices.