The power of preemption is derived from

Preemption

n. the rule of law that if the federal government through Congress has enacted legislation on a subject matter it shall becontrolling over state laws and/or preclude the state from enacting laws on the same subject if Congress has specificallystated it has "occupied the field." If Congress has not clearly claimed preemption, a federal or state court may decide theissue on the basis of history of the legislation (debate in Congress) and practice. Example: Federal standards of meat orother products have preempted state laws. However, federal and state legislation on narcotics control may parallel each other.