CONCURRENT FEDERAL AND STATE POWER --
THE SUPREMACY CLAUSE
Federal law is supreme and conflicting state law is rendered void.
A valid act of congress or federal regulation supersedes any state or local action that actually conflicts with the federal rule -- whether by commanding conduct inconsistent with that required by the federal rule or by forbidding conduct that the fedral rule is designed to foster.
The conflict need not relate to conduct; it is sufficient if the state or local law interferes with acheivement of a federal objective.
A state or local law may fail under the supremeacy clause, even if it does not conflict with federally regulated conduct or objectives, if it appears that congress intended to occupy the entire field, thus precluding any state or local regulation.
-- Federal law may expressly preempt state law, but express preemption clauses will be narrowly construed.
-- If federal law doesn't expressly indicate whether state law should be preempted, the courts will try to deduce congress's intent. If federal laws are comprehensive or a federal agency is created to oversee the area, preemption will often be found.
-- The USSC has stated that in all preemption cases, especially any involving a field traditionally within the power of the states (eg, health, safety, welfare), it will start with the presumption that the historic state police powers are not to be superceded unless that was the clear and manifest purpose of congress.