Overview#Usurpation is the exercise of powers by an agent which have not been delegated to him by the principal.
In a constitutional republic like the United States of America, acts by officials are legitimate only if they are consistent with and based on a United States Constitution, a body of laws which are superior to all subsequent statutes and other acts of officials, which embodies all delegations of power, and which may recognize certain rights to further define the limits on the powers delegated.
It is a fundamental principle that all acts of officials not derived from the delegated powers of the United States Constitution are null and void from inception, not just from the point at which a court may find them unconstitutional. Every person who has an encounter with the acts of officials has the duty not only to obey legitimate official acts, but to help enforce them, but, when there is a conflict among acts of officials, to enforce the superior one, which, when an act of an official is in conflict with the United States Constitution, means enforcing the United States Constitution and not the act in conflict with it.
Judges and other citizens do not decide constitutionality, but discover it, and every person who is involved with any act by an official has a nondelegatable duty to make a determination of the constitutionality of that act. This determination is called constitutional review, and, when exercised by a judge in a case, judicial review.