Generally, when a state law conflicts with a federal law, the state law is preempted. Sources of federal law that will trump state law include the U.S. Constitution, treaties, and laws approved by Congress. Federal courts are empowered to strike down state laws that conflict with federal law.
The framers of the U.S. Constitution were concerned about the possible tyranny that could result from having a centralized government. Therefore, they sought to list the powers of the federal government and leave everything else to the states. The 10th Amendment reserves to state governments all powers "not delegated to the United States by the Constitution, nor prohibited by it to the States."
The Supremacy Clause of Article IV of the Constitution establishes treaties and federal law as the supreme law of the land, voiding any state law that acts in conflict. A state law in conflict with any provision of the Constitution is also void. Subsequent case law has voided state and local laws that conflict with the clear objectives of federal law.
Invalidation of a state law by a federal statute is known as preemption. There are two types of preemption. Express preemption occurs when a provision in a federal statute clearly preempts certain state laws. Implied preemption can occur in one of two ways. It can be field preemption in which a scheme of federal regulation is so broad that it "occupies the entire field," implying that Congress did not intend state law to come into play. Another implied preemption is conflict preemption in which a state law is in direct conflict with a federal law, making compliance with both laws impossible.
Special Law Issues
Under the Constitution, certain state laws are subject to requirements of special consent by the federal government. Congressional consent is required for laws that impose direct state taxes on federal government property or operations, for agreements or compacts made between one state and another, and for taxes on imports. States are prohibited from making laws in certain areas. These include taxes on goods that have "entered the export stream," laws that discriminate against interstate commerce, and laws that impair parties' rights under a previous contract. States also are banned from entering into compacts with foreign governments.
Bill of Rights
The Bill of Rights, which are the first 10 Amendments to the Constitution, were not automatically applicable to the states upon enactment. However, subsequent U.S. Supreme Court decisions have ruled that some of the rights granted in the Bill of Rights are applicable to states, making any state laws that impair those rights invalid. Protections that have been applied to the states include all rights granted by the First Amendment (freedom of speech, press and religion), the Fourth Amendment right to be free from unreasonable searches and seizures, the Fifth Amendment right to be free of self-incrimination and to receive compensation for government takings; and Sixth Amendment's rights to a speedy due and public trial by an impartial jury. Other amendments, such as the 13th Amendment, explicitly apply to the states.