Under the Constitution, federalism divides governing authority between two levels: the federal government holds a defined list of powers spelled out in the text, the states keep everything not given to Washington or forbidden to them, and a set of clauses handles the overlap. Article I lists what Congress may do. The Tenth Amendment reserves the rest to the states or the people. The Supremacy Clause decides which law prevails when the two conflict. Every major dispute about who gets to regulate what — from health care to gun sales to sports betting — is a fight inside this framework.
What the Federal Government Can Do
Article I, Section 8 enumerates the specific tasks Congress can carry out, including coining money, establishing post offices, and declaring war.1Congress.gov. Constitution Annotated – Article I Section 8 Enumerated Powers The premise is simple: if the Constitution doesn’t authorize a federal action, Congress presumptively cannot take it.
Two provisions stretch that list well beyond its literal terms. The Necessary and Proper Clause lets Congress pass any law reasonably needed to execute its listed powers, even when the specific mechanism appears nowhere in the text.2Constitution Annotated. ArtI.S8.C18.1 Overview of Necessary and Proper Clause In McCulloch v. Maryland (1819), the Supreme Court used this clause to uphold the national bank: because Congress has the power to tax, spend, and manage federal finances, chartering a bank was a legitimate means of carrying those powers out.3Legal Information Institute. McCulloch v. State of Maryland
The Commerce Clause has done more to expand federal authority than any other single provision. It gives Congress power to regulate commerce among the states, and in Gibbons v. Ogden (1824) the Court read that reach broadly, covering commercial interactions crossing state lines and treating the power as belonging exclusively to Congress.4Justia. Gibbons v. Ogden, 22 U.S. 1 (1824) Over time the doctrine grew to reach any activity with a substantial economic effect on interstate commerce, which in practice covers most of the modern regulatory state.
The clause also constrains states even when Congress has said nothing. Under the dormant Commerce Clause, states cannot pass laws that discriminate against or excessively burden interstate commerce. A state rule that treats out-of-state businesses worse than local ones is almost always struck down. A neutral rule that still burdens interstate trade is judged by the balancing test from Pike v. Bruce Church (1970): the law survives only if the burden is not clearly excessive compared to the local benefit.5Justia. Pike v. Bruce Church, Inc., 397 U.S. 137 (1970)
What the States Keep
The Tenth Amendment states the default plainly: any power not delegated to the federal government and not prohibited to the states belongs to the states or the people.6Congress.gov. U.S. Constitution – Tenth Amendment That reservation is the constitutional home for most of what state governments actually do.
The core of it is the general police power — the broad authority to legislate for the health, safety, and welfare of residents. States exercise this inherent authority to run elections, structure local governments, license doctors and lawyers, set traffic rules, zone land, and regulate purely intrastate commerce. It isn’t delegated down from Washington. States hold it as sovereigns.
What States Cannot Do
The Constitution also fences off certain state actions. Article I, Section 10 flatly bars states from making treaties, coining money, or passing ex post facto laws that punish conduct retroactively.7Constitution Annotated. Article I Section 10 – Powers Denied States Other restrictions apply unless Congress consents: states cannot impose import or export duties, keep peacetime military forces, or enter compacts with other states or foreign powers on their own. These lines keep states from behaving like independent nations in areas where national uniformity matters.
When Congress Cannot Order States Around
Even within its own enumerated powers, Congress cannot force states to act as its enforcement arm. This is the anti-commandeering doctrine, and it is one of the most important structural protections states have.
The Supreme Court stated the rule directly in New York v. United States (1992): Congress “may not commandeer the States’ legislative processes by directly compelling them to enact and enforce a federal regulatory program.”8Justia. 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, striking down a federal requirement that local sheriffs conduct handgun background checks.9Justia. Printz v. United States, 521 U.S. 898 (1997) In Murphy v. NCAA (2018), the Court applied the same principle to a federal statute that prohibited states from authorizing sports gambling, holding that Congress cannot issue direct orders to state legislatures whether the order is to pass a law or to refrain from passing one.10Justia. Murphy v. National Collegiate Athletic Association, 584 U.S. (2018)
The practical effect is significant. If the federal government wants a policy enforced, federal officials generally have to do it themselves. This is a major reason sanctuary jurisdiction policies exist: federal immigration authorities cannot conscript local police as enforcement personnel. It also keeps political accountability straight, so voters can identify which government is responsible for which policy.
Powers Both Levels Share
Some authority belongs to both governments at once. Congress and state legislatures can each levy taxes, borrow money, and run their own court systems. Article I, Section 8, Clause 1 gives Congress a broad taxing power, and nothing in the Constitution strips states of the same inherent authority.11Constitution Annotated. Article I Section 8 Clause 1 Neither level needs permission from the other to exercise these concurrent powers, which is why federal and state income taxes coexist without either government approving the other.
Much of modern governance runs on cooperation. Environmental regulation is a familiar pattern: Congress sets national minimums through statutes like the Clean Air Act, and states implement and enforce those standards, often going beyond the federal floor. The Necessary and Proper Clause, the Supremacy Clause, and the Tenth Amendment together supply the constitutional footing for this arrangement.
How States Have to Treat Each Other
Federalism also governs the horizontal relationships between states, not just the vertical relationship with Washington.
Full Faith and Credit
Article IV, Section 1 requires every state to respect the public acts, records, and judicial proceedings of every other state. That is what keeps someone from escaping a valid court judgment by crossing a state line. The Supreme Court generally requires states to give conclusive effect to a final judgment from another state’s court, provided the original court had proper authority over the case.12Constitution Annotated. Overview of Full Faith and Credit Clause States have somewhat more room to decline to apply another state’s statute when they have their own law on point, but they cannot categorically shut their courts to claims arising under another state’s law.
Privileges and Immunities
Article IV, Section 2 bars states from discriminating against citizens of other states in fundamental rights. A resident of one state has the right to travel to another and earn a living on substantially the same terms as locals.13Constitution Annotated. Overview of Privileges and Immunities Clause States can still restrict certain political rights, like voting and holding office, to their own residents. But they cannot single out nonresident workers for a special tax or shut out-of-state professionals out of the local market. The protection extends to municipal ordinances as well as state laws.
Which Law Wins When They Conflict
The Supremacy Clause in Article VI settles collisions between the two systems. The Constitution and valid federal laws are the supreme law of the land, and state judges must follow them regardless of what a state constitution or statute says.14Congress.gov. U.S. Constitution – Article VI When a state law conflicts with a valid federal statute, the federal law displaces it. That is preemption.15Constitution Annotated. Preemption Cases
Two limits keep the Supremacy Clause from swallowing state sovereignty. First, preemption applies only when the federal government has acted within its own constitutional authority; a federal law that exceeds Congress’s enumerated powers can be struck down whether or not it conflicts with state law. Second, courts generally presume Congress did not mean to displace state law unless there is clear evidence otherwise. McCulloch v. Maryland also established the flip side: states cannot use their own powers to obstruct valid federal operations, and have no power “to retard, impede, burden, or in any manner control the operations of the constitutional laws enacted by Congress.”3Legal Information Institute. McCulloch v. State of Maryland
Money as Leverage
The federal government’s most effective tool for shaping state policy is not the Supremacy Clause. It is the checkbook. Congress routinely attaches conditions to federal grants, offering states funds only if they adopt particular policies. The classic case is highway funding tied to a minimum drinking age.
In South Dakota v. Dole (1987), the Supreme Court upheld conditional spending but drew boundaries around it. The conditions have to promote the general welfare, be stated clearly enough that states know what they are agreeing to, bear some relationship to a legitimate federal interest, and not themselves be unconstitutional.16Justia. South Dakota v. Dole, 483 U.S. 203 (1987)
The harder limit is coercion. In NFIB v. Sebelius (2012), the Court held that the Affordable Care Act crossed the line when it threatened to strip states of all existing Medicaid funding unless they agreed to a major expansion of the program. That threat, the Court said, amounted to “economic dragooning” that left states without a genuine choice.17Justia. National Federation of Independent Business v. Sebelius, 567 U.S. 519 (2012) The spending power can create incentives. It cannot compel.
Courts as Referee
A system that splits power between two governments needs someone to draw the lines, and the federal courts, ultimately the Supreme Court, fill that role. Every doctrine above — enumerated powers, the Commerce Clause, anti-commandeering, preemption, conditional spending — took its current shape through judicial decisions.
Courts also decide when private citizens can hold states accountable in federal court. The Eleventh Amendment and the broader principle of sovereign immunity generally bar suits against a state without its consent. In Seminole Tribe v. Florida (1996), the Court held that Congress cannot use its Article I powers to override that immunity.18Justia. Seminole Tribe of Fla. v. Florida, 517 U.S. 44 (1996) Congress can abrogate state immunity only under the Fourteenth Amendment’s enforcement power, which is a much narrower route. Whether you can sue a state for violating a federal statute often turns on which constitutional power Congress relied on when it passed the law.