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The preemption doctrine refers to the idea that a higher authority of law will displace the law of a lower authority of law when the two authorities come into conflict. 

Federal Preemption

When state law and federal law conflict, federal law displaces, or preempts, state law, due to the Supremacy Clause of the Constitution. U.S. Const. art. VI., § 2. Preemption applies regardless of whether the conflicting laws come from legislatures, courts, administrative agencies, or constitutions. For example, the Voting Rights Act, an act of Congress, preempts state constitutions, and FDA regulations may preempt state court judgments in cases involving prescription drugs.

Congress has preempted state regulation in many areas. In some cases, such as medical devices, Congress preempted all state regulation. In others, such as labels on prescription drugs, Congress allowed federal regulatory agencies to set national minimum standards, but did not preempt state regulations imposing more stringent standards than those imposed by federal regulators. Where rules or regulations do not clearly state whether or not preemption should apply, the Supreme Court tries to follow lawmakers’ intent, and prefers interpretations that avoid preempting state laws.

State Preemption: Outright, Express, and Implied Preemption

Similar to federal and state laws, state laws will usually prevail when state and local laws are in conflict. As such, the main question courts will seek to answer is whether there is even a conflict.  

Typically 3 types of conflicts exist regarding preemption:

  1. Outright conflict - when an ordinance directly opposes a state law
  2. Express preemption - the state law directly opposes a local power
  3. Implied preemption - this occurs when 1 of 3 things happen:
    1. when the local ordinance prohibits an act permitted by the state legislature
    2. when a local ordinance permits an act prohibited by the state legislature
    3. when there is clear legislative intent that the "field" is preempted by state law
      1. The "field" is usually defined as when there is an extensive scope of state regulation which reflects a state intent to preempt all local regulations that are in a particular area. Occupation of the field is likely to be found when state law addresses an area that has traditionally been a matter of the state (ex: mortgage foreclosure process).

Implied preemption is a controversial doctrine, because this preemption may be significantly harder to prevent than either outright or express preemption. As such, some states have outlawed implied preemption. Further, if a state specifically authorizes an action, then the local government typically cannot restrict the action.

State Preemption: Field Preemption by Implication

Courts have found various ways to determine when a state legislature has preempted a particular field. In Allied Vending Co. v Bowie (1993), the court issued a seven-part test to determine whether a state legislature has preempted a field by implication:

  1. Whether local laws existed prior to enactment of state laws of same subject matter
  2. Whether statutes provide for pervasive admin regs
  3. Whether ordinance regulates an area in which local control has been allowed
  4. Whether state expressly gives concurrent authority
  5. Whether state agency which administers/enforces the law has recognized legal authority to act
  6. Whether the particular aspect of the field sought to be regulated has been addressed by state leg
  7. Whether a 2-tiered regulatory process existing if local laws were not preempted would engender chaos & confusion 


State Preemption: Preempting State Preemption

There are times during which local ordinances will typically preempt state law. According to City of Riverside v. Island Empire Patients Health and Wellness Center Inc. (2013), if significant interests for a particular issue may vary from locality to locality, courts will presume that they should favor the validity of the local ordinances against state preemption, unless the sate statute expressly forbids the ordinance.