What does American federalism require? Most observers agree on a few general principles: federalism in some form is a fundamental ingredient of the U.S. Constitution; at minimum, federalism means that the powers of the federal government are not unlimited; the exercise of those powers must be grounded in text, structure, or practice; and the states should be understood as having a definite and meaningful identity, ranging from co-equal sovereign to regulatory partner. A commitment to federalism requires, in short, that Americans constantly measure their messy legal and political structure against a hazily defined and capacious idea upon which there is little agreement beyond the fact that many of the Founders regarded federalness as one of the nation's essential attributes. Today, federalism means, at a minimum, viewing both the states and the federal government as legitimate sources of legal and political authority, but little consensus exists as to what that general principle of multiplicity should mean in practice. (1)
Where is American federalism to be found in the Constitution? The word is never mentioned in the document itself, in either the 1787 text or the amendments. But commentators have long recognized that the text, structure, and underlying logic of the Constitution assume and endorse a federal system of government. (2) Modern constitutional law typically focuses on three main textual and doctrinal sources of federalism: (1) the enumeration principle (Article I); (2) judicial review of state law by the Supreme Court (Article III plus the Supremacy Clause of Article VI, Clause 2); and (3) the Supremacy Clause itself, especially the requirement that the Constitution, laws, and treaties of the U.S. are the supreme law of the land and bind judges in the states. (3)
A related but distinct potential locus of federalism in both text and doctrine is the Tenth Amendment, which states, "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." (4) Beginning in the 1930s, and gaining new vigor in the 1970s, the Tenth Amendment became the touchstone for the view that federalism means taking the states seriously as sovereigns. (5) Indeed, for many judges and commentators, the mere invocation of the Tenth Amendment amounts to a normative statement about the value of the states in the federal structure and the concomitant limits on federal power. (6) In some cases, the Tenth Amendment is treated as a constitutional guarantee of "the province of state sovereignty" (7) and "local power always existing" in the states; (8) in others, it is "but a truism that all is retained which has not been surrendered." (9)
How do we know what federalism ought to look like today? Following the invalidation of the Child Labor Act in Hammer v. Dagenhart in 1918, and continuing for much of the twentieth century, the paradigmatic federalism question as framed by the Supreme Court was the correct balance between Congress's power to legislate under Article I, on one hand, and the states' large, ill-defined, and perhaps exclusive regulatory domain on the other. (10) In Hammer, Justice Day, writing for the Court, set forth the robust view of the Tenth Amendment that echoed down through decades of case law:
In interpreting the Constitution, it must never be forgotten that the nation is made up of states to which are entrusted the powers of local government.... The power of the states to regulate their purely internal affairs by such laws as seem wise to the local authority is inherent and has never been surrendered to the general government. (11) Justice Holmes's dissenting opinion, in contrast, crystallized the opposing argument from Article I:
I should have thought that the most conspicuous decisions of this Court had made it clear that the power to regulate commerce and other constitutional powers could not be cut down or qualified by the fact that it might interfere with the carrying out of the domestic policy of any State. (12) After 1937, as is well known, the Court adopted an increasingly deferential stance toward congressional regulation under the commerce power; as part of this shift, the Court overturned its decision in Hammer. (13) But the Hammer Court's framing of the debate--inherent state authority over internal affairs on one hand, plenary federal power within a defined Article I sphere on the other--continued to structure the debate for decades. Throughout the 1980s and 1990s, the federalism debate repeatedly returned to the Tenth Amendment side of the Hammer analysis. As part of the "federalism revolution" of the Court under Chief Justice William Rehnquist, several Justices routinely emphasized the role of the Tenth Amendment in limiting Congress's power, most notably its power to regulate interstate commerce. (14) Over and over again, the Court stressed the "boundaries between the spheres of federal and state authority." (15)
The message seemed clear: in order to understand the meaning of federalism, one had to begin from the premise that the domains of state and federal power were fundamentally and forever distinct. Concurrent power was downplayed and sometimes derided as impractical; experiments with overlapping state and federal authority were frequently deemed hazardous to the correct constitutional structure. (16) The state and the federal bailiwicks were regarded as separate spheres, (17) and the Tenth Amendment functioned as a shield around the "States' freedom to structure integral operations in areas of traditional governmental functions," (18) functions that were "essential to [the] separate and independent existence" of the states. (19) The terms of the federalism debate appeared to be set: a majority of the Justices would analyze Congress's Article I powers, especially the commerce power, through the lens of the Tenth Amendment, with a baseline commitment to protecting the special domain of state regulatory authority. (20)
But the terms of the federalism debate have recently changed, with important and potentially far-reaching consequences that have not been fully appreciated--even by the Court itself. The interpretive struggle over the meaning of American federalism has shifted from the Commerce Clause to two textually marginal but substantively important battlegrounds: the Necessary and Proper Clause (21) and, to a lesser extent, the General Welfare Clause. (22) To be sure, the higher-profile commerce power continues to attract an enormous amount of judicial attention and scholarly commentary. (23) But for nearly a decade, the quieter, more structurally ambiguous federal powers listed at the head and foot of Article I have steadily increased in prominence. Today, the battles of judicial federalism are fought not across the well-trampled no-man's-land of the commerce power or the Tenth Amendment, but in the less trafficked doctrinal redoubts of what I term the "shadow powers." This expansion of the battlefield carries important consequences for the meaning of modern federalism. || Beginning with Gonzales v. Raich (24) in 2005 and continuing through United States v. Comstock, (25) National Federation of Independent Business v. Sebelius, (26) and United States v. Kebodeaux (27) the Supreme Court's "federalism revolution" (28) has taken on a new form. The Court's federalism jurisprudence has shifted from its once-typical form of inquiry into the scope of Congress's power to regulate interstate commerce, refracted through the Tenth Amendment, (29) to become an inquiry into the transsubstantive reasons for allowing Congress to regulate at all. This transformation has been especially significant when the Court views Congress as venturing into a domain not explicitly specified in the text of Article I. Analytically, the Justices in the majority in these cases seem to be motivated more by a concern about the expansion of federal regulatory power itself, and somewhat less by a "new federalist" (30)-style belief in a categorical distinction between the proper spheres of state and federal power.
The return to prominence of this pair of ill-defined but foundational provisions of Article I means many things: doctrinal instability, opportunities for creative litigation, opaque or oracular or overly tentative pronouncements by the Justices. But it also provides a moment to think structurally about the Constitution, and perhaps to reach some conclusions about what federalism does and does not require. The resurgence of the Necessary and Proper and General Welfare Clauses in the doctrine is not a sign of intellectual impoverishment or a mere result of crafty litigation strategies; neither is it a retreat to the weedy curtilage of the federalism field. Instead, the return of the shadow powers heralds an opportunity to take up a central question of federalism: Is it possible to conceive of the states as having significance while also recognizing the logic of Holmes's point in Hammer? Holmes's dissent insisted that federalism concerns were irrelevant to determining whether a particular act was within Congress's power. (31) Thus, he maintained, a court need not conduct a separate Tenth Amendment analysis in order to satisfy federalism's demands. (32) According to this view, if a particular act of Congress is within the domain of Congress's power, then one need not keep probing to ask about an amorphous conception of state sovereignty.
Recent doctrine has thus partially revived Justice Holmes's conviction that the best way to approach the federalism question is by inquiring into the scope of Congress's powers. This inquiry increasingly focuses on the shadow powers of Article I: the taxing and spending powers of the General Welfare Clause, and the necessary and proper power. The revival is only partial, however, because a majority of the Justices appears to believe, unlike Justice Holmes, that the...