Preemption is when a federal law displaces a state law. That is, the state law must yield to the federal law and the state law is no longer in effect.

There are three different situations when federal law preempts state law.

First, sometimes a federal law will expressly state that it is preempting a state law.

Second, if a state law and federal law conflict, then the federal law trumps the state law (you saw our example with the state voting law).

Third, Courts also recognize what is sometimes called implied preemption.  Implied preemption provides that in some areas there is no room for state law, even if the state law does not conflict with the federal law.

For example, in Arizona v United States, the Supreme Court determined that Arizona could not enact laws imposing penalties on those who hire unauthorized aliens or imposing penalties on aliens who engage in unauthorized work. According to the majority of Supreme Court justices,  Arizona could not pass these laws because Congress intended for only federal law to apply to unauthorized alien employees.

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