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Preemption is the rule of law that if the federal government through Congress has enacted legislation on a subject matter it shall be controlling over state laws and/or preclude the state from enacting laws on the same subject if Congress has specifically declared it has "occupied the field." Preemption can occur by Congress passing a law, preempting state or local law. If Congress has not clearly claimed preemption, a federal or state court may examine legislative history to determine the lawmakers' intent toward preemption.
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.