Authority in the American system is split: the federal government holds only the powers the Constitution lists for it, and everything else belongs to the states. That is the short version of state versus federal power. The longer version is that federal law wins when it conflicts with state law inside those listed powers, the federal government cannot force states to run its programs, and courts spend a great deal of time deciding which side of the line any given rule falls on.
What the Constitution Actually Divides
Article I, Section 8 lists what Congress can do: regulate commerce among the states, coin money, declare war, and about a dozen other specific functions.1Constitution Annotated. Article I Section 8 The list closes with the Necessary and Proper Clause, which lets Congress pass any law reasonably needed to carry out those listed powers.2Constitution Annotated. Article I Section 8 Clause 18 Courts have read “necessary” to mean appropriate and useful rather than strictly essential, which is why federal authority has grown well beyond what the bare list of powers suggests.
On the other side, the Tenth Amendment reserves every power not handed to the federal government, and not specifically denied to the states, back to the states or the people.3Constitution Annotated. Tenth Amendment That sounds like a clean rule. It is not. Two centuries of argument have gone into deciding where federal authority ends and state authority begins.
When the two do collide directly, the Supremacy Clause in Article VI settles it: the Constitution, federal statutes, and treaties are the “supreme Law of the Land,” and state judges are bound by them regardless of anything in state law to the contrary.4Constitution Annotated. Article VI Clause 2 Federal law wins, but only when it is operating within a power the Constitution actually gave Congress.
When Federal Law Overrides State Law
The Supremacy Clause makes federal law override state law through a doctrine called preemption, which comes in two forms.5Congress.gov. Federal Preemption: A Legal Primer
Express preemption is the straightforward version. Congress writes a statute that says, in so many words, that it displaces state laws on the topic. Fights about express preemption are usually about how far the override reaches, not whether Congress meant to do it.
Implied preemption is harder. Sometimes Congress regulates a field so thoroughly that no room remains for state rules, even ones that would complement federal policy. Other times a state law does not directly contradict a federal law but gets in the way of what Congress was trying to accomplish. Courts have to infer intent from the structure and purpose of the statute, and reasonable judges disagree.
Arizona v. United States (2012) shows how this plays out. Arizona passed a sweeping immigration enforcement law, and the Supreme Court struck down three of its four challenged provisions. Arizona could not create its own crime of being in the country without authorization, because that conflicted with the existing federal registration system. It could not criminalize working without authorization, because Congress had chosen to penalize employers rather than workers. And it could not authorize warrantless arrests based on suspected removability, because that intruded on federal discretion over deportation.6Justia. Arizona v. United States, 567 U.S. 387 (2012) Only one provision survived: a requirement that officers verify the immigration status of people they had already lawfully arrested, which merely facilitated communication with federal authorities.
What Congress Cannot Force States To Do
Federal supremacy has a hard limit. Congress cannot make state governments do federal work. This is the anti-commandeering doctrine, and it flows from the Tenth Amendment.7Constitution Annotated. Anti-Commandeering Doctrine
The rule took shape in New York v. United States (1992). Congress tried to solve radioactive waste disposal by telling states they had to either regulate waste according to federal instructions or take ownership of it themselves. The Court struck that down, holding that Congress cannot commandeer state legislatures by ordering them to enact or enforce a federal program.8Legal Information Institute. New York v. United States, 505 U.S. 144 (1992)
Five years later, Printz v. United States (1997) extended the rule to state executive officers. The Brady Handgun Violence Prevention Act required local law enforcement to conduct background checks on handgun buyers. The Court struck that requirement down, holding that Congress cannot conscript state officers into administering a federal regulatory program any more than it can order state legislatures to pass laws.9Legal Information Institute. Printz v. United States, 521 U.S. 898 (1997)
The most recent big extension is Murphy v. NCAA (2018), which struck down a federal law that prohibited states from authorizing sports betting. The Court rejected any meaningful difference between forcing a state to act and forbidding a state from acting. Either way, Congress is dictating what a state legislature may do, and that violates the same structural principle.10Supreme Court of the United States. Murphy v. National Collegiate Athletic Assn. States promptly began legalizing sports betting on their own terms.
How Washington Gets States To Act Anyway
Congress cannot order a state to legislate, but it can pay a state to do it. By attaching conditions to federal grants, Congress steers state policy in areas where it has no direct regulatory authority.
The National Minimum Drinking Age Act of 1984 is the classic move. The law did not set a national drinking age. It told states they would lose a portion of their federal highway funding if they allowed anyone under 21 to purchase alcohol.11National Highway Traffic Safety Administration. Fact Sheet Minimum Drinking Age Laws Every state complied. The Supreme Court upheld this approach in South Dakota v. Dole (1987), ruling that Congress can use financial incentives to encourage state behavior if the conditions relate to a legitimate federal interest. But the Court flagged a limit: at some point financial pressure stops being encouragement and starts being coercion.
That limit was reached in NFIB v. Sebelius (2012), when the Court ruled that the Affordable Care Act’s Medicaid expansion crossed it. The ACA required states to extend Medicaid to a much larger population, and states that refused stood to lose not just the new expansion funding but all of their existing Medicaid money. Because Medicaid accounts for over 20% of the average state budget, the threatened loss amounted to what the Court called “economic dragooning” that left states no real choice.12Justia. National Federation of Independent Business v. Sebelius, 567 U.S. 519 (2012) Congress could offer the expansion funds to willing states, but it could not threaten to pull existing funding from states that declined.
Unfunded Mandates
Conditional grants at least come with money. Unfunded mandates are the less popular cousin: federal requirements that states must carry out at their own expense. Congress passed the Unfunded Mandates Reform Act in 1995 to slow the practice. The law requires federal agencies to prepare a cost-benefit analysis and consider less burdensome alternatives before imposing any rule that would cost state, local, or tribal governments $100 million or more in a single year.13United States Environmental Protection Agency. Summary of the Unfunded Mandates Reform Act The Act also requires agencies to consult with elected state and local officials before finalizing significant intergovernmental mandates. It has reduced the frequency of unfunded mandates without eliminating them.
Where the Commerce Clause Line Sits Now
No provision has done more to expand federal power than the Commerce Clause, which lets Congress regulate commerce “among the several States.” The Court’s reading of that phrase has swung dramatically. In Wickard v. Filburn (1942), a farmer growing wheat on his own land for his own livestock argued he was not engaged in interstate commerce. The Court disagreed, holding that even purely local activity falls within Congress’s reach if, taken together with similar activity by others, it substantially affects the national market.14Justia. Wickard v. Filburn, 317 U.S. 111 (1942) Under this aggregate-effects test, very little economic activity is truly beyond federal regulation.
The pendulum swung back in United States v. Lopez (1995), the first case in decades where the Court said Congress had overstepped. The Gun-Free School Zones Act made it a federal crime to carry a firearm near a school. The Court held that gun possession in a school zone is not economic activity and has no substantial connection to interstate commerce.15Justia. United States v. Lopez, 514 U.S. 549 (1995) There are outer limits to federal power, even under the most expansive clause in the Constitution.
Suing a State
One boundary catches people off guard. The Eleventh Amendment generally bars private citizens from suing a state in federal court without the state’s consent. The text bars suits against a state by citizens of other states, but Hans v. Louisiana (1890) extended the reach to also bar suits by a state’s own citizens.16Constitution Annotated. General Scope of State Sovereign Immunity
In Seminole Tribe v. Florida (1996), the Court ruled that Congress cannot use its Article I powers to override state sovereign immunity. So Congress can pass a law regulating some aspect of state behavior, but it often cannot give private individuals the right to enforce that law against a state in federal court. Congress can get around this only by acting under Section 5 of the Fourteenth Amendment, which authorizes legislation to enforce equal protection and due process rights. If you are trying to hold a state government accountable for violating a federal statute, that distinction can decide your case.
Where This Plays Out Today
The doctrines above are not abstractions. They decide which rules apply to you depending on the state you live in.
Marijuana
Marijuana remains a Schedule I controlled substance under the federal Controlled Substances Act, which classifies it as having no accepted medical use and a high potential for abuse.17Congress.gov. Legal Consequences of Rescheduling Marijuana Roughly 40 states allow medical cannabis, and 24 states have legalized recreational use. Federal prosecutors have generally chosen not to pursue individuals complying with state law, but the conflict is real: a marijuana business that is fully legal under state law is technically committing a federal crime, which creates problems with banking, taxes, and insurance. A rescheduling process to move marijuana from Schedule I to Schedule III was proposed in 2024 but had not been finalized as of late 2025.
Healthcare
Healthcare may be the most tangled of all. The Affordable Care Act expanded federal influence by creating insurance marketplaces, setting coverage requirements, and extending premium subsidies to households earning between 100% and 400% of the federal poverty level.18U.S. Department of Health and Human Services. About the Affordable Care Act But after NFIB v. Sebelius struck down the mandatory Medicaid expansion, whether to expand became a state-by-state choice, and not all states have opted in.12Justia. National Federation of Independent Business v. Sebelius, 567 U.S. 519 (2012) States also retain broad authority over licensing healthcare professionals, regulating insurance markets, and setting public health policy. The result is a patchwork that varies significantly by geography.
Firearms
The Supreme Court’s 2022 decision in NYSRPA v. Bruen struck down New York’s requirement that concealed-carry applicants show a special need for self-defense. The Court held that states may require objective criteria like background checks, but they cannot use subjective, discretionary evaluations to decide who gets to carry. Any firearm regulation must now be justified by reference to the historical tradition of firearms regulation in the United States. The decision immediately affected half a dozen states with similar permitting systems and set off a wave of litigation over other gun laws.
Environmental Regulation
Environmental protection was historically a state responsibility, but the creation of the EPA and the major federal laws of the 1970s shifted much of the regulatory authority to Washington. The Clean Air Act directs the EPA to set national air quality standards while requiring states to develop implementation plans to meet them.19United States Environmental Protection Agency. Summary of the Clean Air Act The Clean Water Act follows a similar structure, setting federal pollution standards and relying on state permitting programs for enforcement.20US EPA. Summary of the Clean Water Act Federal and state agencies negotiate constantly over how strict the standards should be and how much flexibility states get in meeting them, and that negotiation is the modern face of federalism.