What is nullification?
Though it may seem like a new theory with the opposition to President Obama’s health care plan, cap and trade and other intrusive, unconstitutional laws, nullification is far from being new.
The legal principle of nullification can be simply defined as an act by the states (or less frequently by individuals) to resist and refuse to enforce unconstitutional federal laws.

Critics may cry “They told us so” as the cost increases from Obamacare become more clear
Photo/Florida Tenth Amendment Center
Nullification in our country probably dates back to when the colonists nullified laws made by the King in the pre-constitutional era.
When has nullification been used in our post-constitution history?
Nullification has been used numerous times throughout are history to check the federal government. The most commonly cited cases are the Kentucky and Virginia Resolves.
Certainly the biggest proponent of nullification was Thomas Jefferson (he actually coined the word) when he authored anonymously the 1798 Kentucky Resolve.
This of course was in response to John Adam’s despicable Sedition Act which outlawed criticism’s of the President. Interestingly, the act was set to expire on the last day of Adam’s term. He certainly wouldn’t want the atrocious law being enforced on him.
As Dr. Tom DiLorenzo once put it, it made it a crime to publish “false, scandalous, and malicious writing against the government or its officials.” Of course, the government itself would solely decide what constituted improper and illegal speech, as was the case in the Soviet Union and all other totalitarian states during the twentieth century.
And the “father of the Constitution”, James Madison followed this the next year with the Virginia Resolves.
Both Madison and Jefferson stated that the states constitutionally had the right to refuse to comply with such blatantly unconstitutional actions, but also to prevent the feds from enforcing the unconstitutional laws within their states.
Not only did the Sedition Act violate the First Amendment but as Dr. Thomas Woods explains, the constitutional justification for nullification of unconstitutional laws is the Tenth Amendment; “The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.”
The 10th Amendment is critical to understanding both the limitations of the federal government as well as the unlimited nature of the response appropriate from the states and the people.
The act of nullification was used by the states numerous times after this to include nullifying the embargo act in 1809 and some New England states essentially seceding from the union so not to participate in the undeclared War of 1812 to name a couple.
Nullification has even been implemented as recently as this past decade with the Real ID Act of 2005. At least half of the states formally said they wouldn’t comply with the law that would tell states how to issue driver’s licenses. Even though the law is still on the books, according to Dr. Woods “resistance was so widespread, the federal government has, in effect, given up trying to enforce it.”
Nullification was and is a necessary tool for the states and individuals to keep check on Washington.
Required reading for more details:
The Tenth Amendment Center web site
Also you may want to check out Nullify Now!