No one "declares" a federal law. Once Congress has passed the Bill and forwarded it for the Presidents signature - once he signs it it becomes law.
nullify
The states must obey the rules and laws of Federal Government.
In the United States, the power to declare a law unconstitutional lies with the judicial branch of government, specifically the federal courts, including the Supreme Court.
that individual states may declare federal law null and void
Nullification
That would be a claim of seperation of power.
supreme court ^O^
Congress cannot declare laws unconstitutional. The Judiciary Branch may declare a law unconstitutional only if it conflicts with some provision of the State or Federal Constitution. The Supreme Court can rule a law to be unconstitutional, but Congress, along with the States, can only amend the Constitution.
Federal law, and codified by all states, declare that this is the legal age for aclochol consumption.
The right of a state to "NULLIFY, or DECLARE VOID, ANY FEDERAL LAW WITHIN ITS BOUNDARIES."
One Constitutional power specifically delegated to the federal government is the power to declare war.
The Doctrine of Nullification held that states had the right to declare null and void any federal law they deem unconstitutional.