The doctrine of "severability" permits a court to excise the unconstitutional portion of a partially unconstitutional statute in order to preserve the operation of any uncontested or valid remainder. Severability figures centrally in a broad array of constitutional litigation, including recent litigation over the "individual mandate" provision of the Patient Protection and Affordable Care Act. Nevertheless, the doctrine remains underexplored. In particular, no commentator has thoroughly examined choice-of-law rules pertaining to its application. This Article aims to fill that void. The Article contends that in recent decisions the Supreme Court has quietly established the severability of state statutes in federal court to be a matter of general federal common law, and that this doctrine is not only inconsistent with dozens of cases decided since Erie Railroad Co. v. Tompkins, but also displaces a large body of diverse state law without constitutional authorization or a supporting federal interest. The new doctrine thus challenges standard accounts of the limits of federal common law and calls into question the contemporary vitality of Erie's principle of judicial federalism. The Article closes by proposing an alternative that would harmonize the precedent, help to revitalize Erie, and honor the bounds of Article III judicial power.
Consider the following problem: A federal court has declared a single provision in a large state statute to be facially unconstitutional, and must decide what effect that declaration has on the rest of the statute. If it is possible to excise and discard the unconstitutional part, then the declaration's effect will be limited and the remainder will continue in force. If excision is not possible, however, the unconstitutional part will in effect bring down the entire statute with it. The stakes may be high. But under whose law should the court decide whether to engage in this form of statutory surgery? Is it the federal law of the reviewing court? Or is it the potentially different law of the state whose enactment is under review? The question is one of vertical choice of law with respect to the doctrine of "severability." And within the last few years the Supreme Court has quietly developed a surprising answer-an answer that has general federal common law partially displace a large body of what can be materially different state common law, without specific constitutional authorization or any supporting federal interest. It is an answer that is reminiscent of the era of Swift v. Tyson,1 and that calls into question the traditional limits of federal common law and the contemporary meaning of Erie Railroad Co. v. Tompkins.2
The doctrine of severability holds that upon finding an application or textual component of a statute to be unconstitutional, a court may, in appropriate circumstances, excise the unconstitutional part rather than declare the entire statute invalid.3 The basic rationales for severance are that it can minimize judicial interference with legislative lawmaking, honor legislative intent, and promote legislative innovation by lowering the stakes of a ruling of partial unconstitutionality.4 The doctrine is frequently relevant because any holding that a statute is partially invalid will give rise to questions concerning what to do with the valid remainder. And the doctrine is powerful because the viability of large statutory schemes can hinge entirely on whether an unconstitutional component is severable.5
The Supreme Court's severability jurisprudence spans from the late 1800s to the 2012 decision National Federation of Independent Business v. Sebelius.6 As others have recounted in detail, the doctrine's content has varied significantly over time. 7 Whether statutes are to be presumed severable, for example, has changed repeatedly.8 Whether the Court honors the plain text of severability clauses has varied. 9 And precedent has historically …