When state law and federal law genuinely clash, federal law wins. That is the short answer to how conflicts between state and federal laws are resolved, and it comes straight from the Supremacy Clause in Article VI of the Constitution, which makes the Constitution, federal statutes, and treaties the “supreme Law of the Land.”1Cornell Law School / LII. Article VI U.S. Constitution The longer answer matters, though, because most disputes are not clean conflicts. Courts use a doctrine called preemption to decide whether a real conflict exists, and federal power has firm limits that keep states from being turned into arms of the federal government.
Two things have to be true before the Supremacy Clause does any work. The federal law must itself be constitutional, meaning Congress had the power to pass it. And there has to be an actual clash. Overlap alone is not conflict.
Preemption: The Framework Courts Actually Use
Preemption is how judges decide whether a federal law has knocked out a state law. It shows up in two broad forms.
Express Preemption
Sometimes Congress says so directly. A federal statute will include language stating that states may not regulate in a particular area. Federal cigarette labeling law works this way: states cannot impose their own health-warning requirements on cigarette packaging or advertising content, though they remain free to restrict where and when cigarette ads appear.2Office of the Law Revision Counsel. 15 USC 1334 – Preemption When Congress writes a preemption clause, courts start there.
Implied Preemption
When Congress does not spell it out, courts look at the structure and purpose of the federal law. This takes two main forms.
Field preemption applies when federal regulation is so comprehensive that it leaves no room for state involvement. Immigration is the standard example. In 2012, the Supreme Court struck down key provisions of Arizona’s immigration enforcement law, holding that federal regulation of alien registration was so thorough that even complementary state rules could not stand.3Justia Law. Arizona v United States 567 US 387
Conflict preemption splits into two situations. Impossibility preemption applies when a person cannot comply with both laws at once. Obstacle preemption applies when a state law, even if not flatly contradictory, undermines what Congress was trying to achieve. A state that permitted a lower safety standard than a federal minimum would fall into the second category.
Preemption doctrine also reaches agency regulations, not just statutes. Rules issued by federal agencies like the FDA, EPA, or Department of Transportation can displace state law when Congress has given them authority to do so.4Cornell Law School. Preemption
The Presumption Against Preemption
Preemption analysis is not neutral. In areas where states have traditionally exercised authority, courts start with a presumption that Congress did not intend to displace state law. The rule, established in Rice v. Santa Fe Elevator Corp., holds that “the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress.”5Justia Law. Rice v Santa Fe Elevator Corp 331 US 218 Where a statute’s preemptive reach is ambiguous, courts favor the reading that preserves state authority.4Cornell Law School. Preemption
The practical effect is that the party arguing for preemption carries the burden. Most disputes involve gray areas, and in those cases, silence or ambiguity from Congress usually helps the state.
Floor Versus Ceiling
Not every federal standard displaces state action. The distinction is whether Congress set a floor or a ceiling.
A floor sets a minimum that every state must meet, while leaving states free to go further. The federal minimum wage sits at $7.25 per hour, and many states have set higher rates for workers within their borders without any conflict, because the higher state rates reinforce the federal purpose instead of undercutting it.6U.S. Department of Labor. State Minimum Wage Laws
A ceiling works differently. It displaces state regulation entirely, so states cannot go either below or above the federal requirement. Congress preempted medical device safety standards this way: for devices that have gone through FDA approval, states cannot impose requirements “different from, or in addition to” the federal standards.7eCFR. Part 808 Exemptions from Federal Preemption of State and Local Medical Device Requirements Even a stricter state safety rule is preempted, because the goal is nationwide uniformity.
To tell which type applies, look for a savings clause in the federal statute. A savings clause preserves state authority to regulate more strictly and signals a floor rather than a ceiling.
What the Federal Government Cannot Do
The Supremacy Clause gives federal law the last word in a real conflict, but it does not give Washington unlimited authority over states. The Tenth Amendment reserves to the states and the people the powers the Constitution does not give the federal government.8Cornell Law School Legal Information Institute. Tenth Amendment From that reservation comes the anti-commandeering doctrine.
Congress cannot order state governments to carry out federal programs. It cannot direct a state legislature to pass a particular law, and it cannot conscript state officials to enforce federal regulations.9Cornell Law School. Anti-Commandeering Doctrine The Supreme Court has called such commands “fundamentally incompatible with our constitutional system of dual sovereignty.”
In Murphy v. National Collegiate Athletic Association (2018), the Court struck down a federal law that prohibited states from authorizing sports gambling. Telling a state legislature what it may or may not authorize, the Court reasoned, is just as much commandeering as ordering it to pass a law. States could set their own rules on sports betting after that ruling.9Cornell Law School. Anti-Commandeering Doctrine
Congress retains one powerful workaround: conditional spending. It can offer federal funds on the condition that a state adopts a particular policy, which is how the national 21-year-old drinking age effectively became universal without a direct federal mandate. The line is between incentive and coercion.
When the Same Conduct Violates Both
If a single act breaks both federal and state law, both governments can prosecute you separately without triggering double jeopardy. This is the dual sovereignty doctrine, and it surprises most people who run into it.
Each government draws its authority from a different source, so a crime against one sovereign’s laws is legally a distinct offense from a crime against the other’s. The Supreme Court reaffirmed this in Gamble v. United States (2019), holding that a defendant prosecuted by Alabama for illegal firearm possession could also be prosecuted federally for the same conduct.10Justia Law. Gamble v United States 587 US
The doctrine has a limit. Two prosecutors within the same sovereign, like a city attorney and the state attorney general, cannot prosecute someone twice for the same offense, because they draw power from the same government. State followed by federal, or federal followed by state, is allowed.
Cannabis: The Conflict That Has Not Been Resolved
Cannabis is the sharpest current example of federal and state law pulling in opposite directions. Marijuana remains a Schedule I controlled substance under federal law, alongside heroin and LSD. Meanwhile, most states have legalized it for medical use, recreational use, or both. A cannabis business can be fully licensed under state law and still be violating federal law.
The federal government’s authority to enforce its marijuana ban even in states that have legalized cannabis was confirmed by the Supreme Court in Gonzales v. Raich, which held that Congress’s Commerce Clause power reaches local cultivation and use. Federal enforcement against state-legal operations has been inconsistent in practice, shifting with administration and prosecutorial priorities.
The consequences are concrete. Section 280E of the Internal Revenue Code bars any business trafficking in Schedule I or II substances from deducting ordinary business expenses.11Office of the Law Revision Counsel. 26 USC 280E – Expenditures in Connection with the Illegal Sale of Drugs While most companies subtract rent, salaries, and advertising from revenue before paying tax, cannabis businesses can deduct only the direct cost of their goods. Effective tax rates can exceed 70%. Banks, meanwhile, risk criminal liability under federal money laundering statutes if they serve cannabis businesses, forcing many operators to run on cash.
As of late 2025, a Department of Justice rule was pending that would reschedule marijuana from Schedule I to Schedule III, which would remove the Section 280E penalty. A December 2025 executive order directed the Attorney General to complete the rescheduling process as quickly as possible.12The White House. Increasing Medical Marijuana and Cannabidiol Research Until that is finalized, the conflict stands.
How These Fights Actually Reach a Decision
Preemption disputes end up in court. Usually a party in a lawsuit argues that a state regulation should not apply because it conflicts with a federal statute, regulation, or constitutional provision. These cases can be filed in state or federal court. Someone facing immediate harm from a state law can ask for a preliminary injunction, which temporarily blocks enforcement while the case proceeds.
The final word belongs to the U.S. Supreme Court. A preemption case can travel up through either the federal or the state court system, and under federal law the Supreme Court can review a final judgment from a state’s highest court when the case challenges the validity of a federal or state statute on constitutional grounds.13Office of the Law Revision Counsel. 28 USC 1257 – State Courts Certiorari Through judicial review, the Court can invalidate any law, state or federal, that violates the Constitution. That power is what makes the whole framework enforceable.