Nullification begins with the axiomatic point that a federal law that violates the Constitution is no law at all. It is void and of no effect. Nullification simply pushes this uncontroversial point a step further: if a law is unconstitutional and therefore void and of no effect, it is up to the states, the parties to the federal compact, to declare it so and refuse to enforce it. It would be foolish and vain to wait for the federal government or a branch thereof to condemn its own laws. Nullification provides a shield between the people of a state and an unconstitutional law from the federal government.
Thomas E. Woods, Jr., Nullification: How to Resist Federal Tyranny in the 21st Century (Regnery, 2010), p. 3.