Site: nolo.com
... 1) The principle that a federal law supersedes a state law, or a state law supersedes a local law, when ... both governments have made laws on the same subject. ... Preemption can be found when:
the superseding federal or state law expressly preempts conflicting state ... or local laws, or
the superceding federal or state law is comprehensive and occupies the field, or a ... conflicting state or local law frustrates the purpose of the superceding law.
2) The right to purchase ...