In the United States federal statutes can limit the state's powers by invalidating conflicting state and local laws. One way that this can be achieved is by Congress passing a law, preempting state or local law. State powers can also be limited by the Supremacy Clause.
Article VI, section 2 of the United States Constitution states: 'This Constitution, and the Laws of the United States which shall be made in Pursuance there of: and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary not withstanding.' This clause is commonly referred to as the 'Supremacy Clause.' The Supremacy Clause says that federal law is the 'Supreme Law of the Land.' This means that if a state or local law is in conflict with a federal statute, the federal statute will preempt the state or local law.
Two situations where preemption claims might arise: express preemption and implied preemption.
Express preemption occurs where Congress says within the statute 'we hereby preempt.' Here, federal laws are explicitly precluding state and local regulations.
Implied preemption has, within itself, three sub-categories: conflicts preemption, preemption because state law impedes the achievement of a federal objective, and preemption because federal law occupies the field.
Conflicts preemption is where it is impossible to comply with both the federal statute and the state or local law. In this situation, the federal statute must be followed. It is, however, appropriate to have two laws, one federal and one state, that differ. The federal law, in this case, may be a minimum standard, while the state enacts a law to be more strict. State law, therefore, would not be preempted. Preemption would only occur if the federal and state laws were mutually exclusive.
The second type of implied preemption is preemption because state law impedes the achievement of a federal objective. This type of preemption occurs when a state or local law interferes with a goal or objective Congress was trying to attain with a federal statute. The purpose of each law must be determined and compared to each other. If both laws are trying to achieve the same goal, federal law will preempt the state or local regulation.
The final type of implied preemption is preemption because federal law occupies the field. In this situation, one must look at Congress's intent, and whether the federal law was meant to be exclusive in that area. The most common examples are in areas of foreign policy and immigration.