Preemption means that, as a result of the exercise of federal authority in a given regulatory area, preexisting concurrent state authority to regulate that same area comes to an end. Although earlier in this century, the Supreme Court viewed preemption as an automatic consequence of federal entry into a regulatory field, the modern view understands preemption as a discretionary constitutional power of Congress (and, by delegation, of federal administrative agencies), the exercise of which requires manifestation of an intent to preempt the states. This change is reflected in the modern presumption against preemption, a presumption that concurrent state authority survives the exercise of federal regulatory power.
Congress has the power to rebut this presumption and preempt state authority regardless of the content of any state law—even if, for example, existing state law is substantively identical to the federal scheme of regulation or there is no state law at all in the relevant area. By exercising its power of preemption, the federal authority elects to monopolize a regulatory field, in whole or in part.
The fact that conflict between the contents of state and federal law is not necessary for preemption to occur distinguishes preemption from the related but separate principle of the supremacy of federal law enshrined in the supremacy clause of the Constitution. This principle means that a valid federal law trumps an otherwise valid state law if (and only if) the two conflict with each other. Unlike preemption, the operation of the supremacy clause does not end general state authority in an area, but results in the trumping of a particular state law by a particular federal law where the two conflict. So, for example, a new state law passed to avoid the conflict would not be prevented from taking full legal effect. Moreover, once Congress has exercised its power of preemption, no state authority survives in the preempted field so that there can no longer be a valid state law in conflict with the federal one to trigger operation of the supremacy clause. The latter operates only where concurrent state authority exists and has not already been preempted.
Preemption thus not only differs from the principle of supremacy but also constitutes a broader inroad on state authority than the latter. One important implication of this point is that the (greater) federal power to preempt state authority cannot derive from the (lesser) principle of supremacy contained in the supremacy clause—as is widely assumed—but must have some other source in the Constitution.
The Court's "preemption doctrine" holds that Congress can exercise its preemption power either expressly—by clearly stating in the legislative text whether, and to what extent, state authority survives its new regulatory scheme—or impliedly. Notwithstanding the absence of any explicit statement on the issue, Congress's intent to preempt may be implicitly contained in a statute's structure and purpose.
The Court has recognized two types of such implied preemption. The first, termed "field preemption," is, according to Rice v. Santa Fe Elevator Corp. (1947), where the scheme of federal regulation is "so pervasive as to make reasonable the inference that Congress left no room for the States to supplement it." The second, known as "conflict preemption," is, according to Hines v. Davidowitz (1941), where compliance with both federal and state regulation is a physical impossibility or where state law "stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress."
There is obviously significant tension between the general notion of implied preemption and the modern presumption against preemption. Because the presumption means the states are not preempted unless that is "the clear and manifest purpose of Congress," it might well be thought that such purpose can, or at least should, only be expressly manifested. Even assuming the general legitimacy of inferring preemptive intent, however, there are specific problems with both types of implied preemption. First, the doctrine of field preemption is arguably too blunt an interpretive instrument and ignores alternative readings of congressional intent that might be reached by an unmediated interpretation of the relevant statute. The proposition that in itself the pervasiveness or comprehensiveness of a scheme of federal regulation makes reasonable an inference of preemptive intent may be both overinclusive and underinclusive with respect to any particular scheme. Overinclusive because, notwithstanding its comprehensiveness, Congress may (1) intend that the states be permitted to supplement it, (2) not have considered the issue, or (3) have considered it but not reached agreement. Underinclusive because Congress may intend to preempt state law while leaving the field relatively, or even entirely, unregulated by any level of government.
Second, the doctrine of conflict preemption is either a contradiction in terms or it similarly imputes to Congress an intent that may be highly questionable in practice. On its face, the proposition that state law is preempted if it conflicts with federal law appears to express the most basic confusion between the distinct principles of preemption and supremacy. The trumping of an otherwise valid state law by supreme federal law is not an instance of preemption at all but a straightforward operation of the principle of supremacy. It neither ends concurrent state authority nor turns on congressional purpose. If, however, the claim is the more subtle one that such a conflict provides the necessary evidence for implying congressional intent to preempt, this generally appears to be an unlikely piece of statutory interpretation. Where preexisting state regulation is followed by a conflicting federal statute that neither "occupies the field" nor contains an express preemption provision, it seems more reasonable to infer that Congress only intended to trump the relevant state statutes without divesting the states of their concurrent legislative authority in that field.
Engdahl, David 1987 Constitutional Federalism in a Nutshell, 2nd ed. St. Paul, Minn.: West.
Gardbaum, Stephen 1994 The Nature of Preemption. Cornell Law Review 79:767–815.