American federalism is the constitutional arrangement by which governmental authority in the United States is divided between a national (federal) government and the governments of the individual states. The system is established by the U.S. Constitution, which grants enumerated powers to the federal government, reserves other powers to the states or the people via the Tenth Amendment, and defines rules for the interaction between the two levels. American federalism is distinct from both a unitary system - in which a central government holds supreme authority - and a confederation - in which member states retain ultimate sovereignty. The precise boundary between federal and state authority has been contested since the Constitution's ratification in 1788 and remains a live legal and political question.

The structure emerged from the failures of the Articles of Confederation (1781-1789), which created a weak central government unable to tax, regulate commerce, or enforce its own laws. The Constitution replaced it with a stronger federal framework, but the degree of that strength was immediately contested. The founding generation disagreed sharply over how much power the new national government was intended to hold, a debate recorded in the Federalist Papers, the Anti-Federalist writings, and the ratification conventions of the states. For the longer arc of this dispute, see American Federalism - History.

Several structural features of American federalism remain the subject of ongoing legal, political, and scholarly dispute:

• The Commerce Clause (Article I, Section 8) grants Congress power to regulate interstate commerce. Courts have interpreted this clause expansively at some periods and more narrowly at others, with significant consequences for federal regulatory reach over economic activity, labor, environment, and health.
• The Spending Power allows Congress to attach conditions to federal funds distributed to states. Critics argue this creates effective federal mandates without formal legislation; defenders argue the arrangement is voluntary because states may decline the funds.
• Preemption occurs when federal law supersedes conflicting state law under the Supremacy Clause. The scope of implied preemption - where Congress has not explicitly addressed the question - is frequently litigated.
• The Tenth Amendment reserves to the states or the people powers not delegated to the federal government. Its force as a substantive limit on federal action, versus a mere truism, is disputed among constitutional scholars and judges.
• Cooperative federalism describes arrangements in which federal and state governments jointly administer regulatory programs (as in Medicaid or environmental law). Critics on both left and right have questioned whether such arrangements obscure accountability.
• Nullification and interposition - the doctrines that states may refuse to enforce or may interpose against federal laws they deem unconstitutional - lost judicial and political standing after the Civil War but have seen rhetorical revival in debates over immigration enforcement, marijuana legalization, and gun control.

The balance of federal and state authority has shifted substantially over U.S. history, most dramatically during Reconstruction, the New Deal, and the civil rights era. The direction and legitimacy of each of those shifts remains contested. See American Federalism - Debate.

There is broad legal consensus that the Constitution establishes a system of dual sovereignty in which neither the federal government nor the states are wholly subordinate to the other, and that neither may “commandeer” the other's governmental machinery (see New York v. United States, 1992; Printz v. United States, 1997). Beyond that structural baseline, scholarly and judicial consensus fractures on nearly every significant boundary question. See American Federalism - Constitutional Law Consensus.

• Nationalist / broad federal power - The federal government's authority, particularly under the Commerce Clause and Spending Power, should be read broadly to address national problems that states cannot solve individually. Associated with the New Deal constitutional settlement and much of the modern administrative state. Read more.

• States' rights / dual federalism - The Constitution established a limited federal government of enumerated powers, with the states retaining broad authority over domestic affairs. Federal expansion into areas such as education, land use, and local law enforcement is constitutionally dubious and practically harmful. Read more.

• Competitive federalism - A federal structure in which states actively compete for residents and businesses on the basis of differing laws and tax regimes produces beneficial policy experimentation and constrains governmental overreach. Read more.

• Progressive federalism - Federal power should be used affirmatively to secure individual rights and reduce inequality; state autonomy claims have historically been used to resist civil rights and labor protections and should be viewed with suspicion on those grounds. Read more.

• Libertarian decentralization - Both federal and state governments have expanded well beyond legitimate bounds; decentralization is desirable chiefly as a step toward reducing the overall scope of government, not as an end in itself. Read more.

• New Federalism (Reagan-era and successors) - Beginning in the 1980s, a political movement sought to return enumerated functions to the states through block grants, regulatory rollback, and judicial appointments sympathetic to limits on federal power. Read more.

• American Federalism - History
• American Federalism - Debate
• American Federalism - Constitutional Law Consensus
• Separation of Powers
• Constitution of the United States
• Tenth Amendment
• Commerce Clause
• Nullification
• Reconstruction

1. U.S. Constitution, Art. I § 8; Art. VI § 2; Amend. X.
2. Alexander Hamilton, James Madison, and John Jay, The Federalist Papers (1787-1788), particularly Federalist No. 39 (Madison) and No. 45 (Madison) on the division of powers.
3. Herbert J. Storing, ed., The Complete Anti-Federalist (University of Chicago Press, 1981).
4. McCulloch v. Maryland, 17 U.S. 316 (1819) - foundational ruling on implied federal powers and state limits.
5. United States v. Lopez, 514 U.S. 549 (1995) - reestablishing outer limits of the Commerce Clause.
6. New York v. United States, 505 U.S. 144 (1992); Printz v. United States, 521 U.S. 898 (1997) - anti-commandeering doctrine.
7. South Dakota v. Dole, 483 U.S. 203 (1987) - federal spending conditions and state autonomy.
8. Edward S. Corwin, “The Passing of Dual Federalism,” 36 Virginia Law Review 1 (1950) - influential account of the New Deal shift.
9. Erwin Chemerinsky, Constitutional Law: Principles and Policies, 6th ed. (Wolters Kluwer, 2019) - standard nationalist scholarly treatment.
10. Randy E. Barnett, Restoring the Lost Constitution (Princeton University Press, 2004) - influential originalist/libertarian critique.