any states took legislative action to prevent the enforcement of ACA within their borders by either nullifying the "individual mandate" portion of ACA or invalidating the law in its entirety. However, the legitimacy of these legislative actions is questionable, given the potential conflict between the Supremacy Clause of the US Constitution, which dictates that federal law preempts state law when the two conflict, and the Tenth Amendment, which reserves for the states those powers not granted to the federal government.
The first legislative challenge to enforcement of ACA began in Virginia on March 11, 2010, twelve days before ACA became law. The Virginia Health Care Freedom Act bans any federal mandate that individuals must purchase health insurance, and provides that no individual shall be held liable for refusing to do so. Following Virginia's opposition, Arizona and Oklahoma voters approved state constitutional amendments prohibiting enforcement of the individual mandate, while Colorado rejected a similar bill on the same day.
On November 8, 2011, Ohio voters passed a referendum [PDF] that forbids enforcement of the ACA's individual mandate and prohibits the imposition of fines on individuals for failing to purchase health insurance. As of November 2011, Alabama, Florida, Georgia, Louisiana, Kansas, Missouri, New Hampshire, Utah, and Wyoming have also passed legislation that challenged the ACA in some form.
Several states also began pursuing legislation that would create an interstate Health Care Compact (HCC) that, if adopted by enough states and approved by the US Congress, would give the states primary authority to regulate health care and insurance. Spearheaded by the Health Care Compact Alliance, the final language of HCC was released [PDF] on February 23, 2011. Although such compacts have traditionally been used to address interstate conflicts such as border disputes and conflicting state agency guidelines, Georgia, Missouri, Oklahoma, and Texas adopted the HCC by November 2011. Similar proposals were also passed by the legislatures of Arizona and Montana and vetoed by their respective governors.
In addition to legislative initiatives, there have also been direct gubernatorial actions to invalidate the law. On April 20, 2011, Idaho Gov. CL "Butch" Otter issued an executive order barring his state from establishing any program or rule implementing provisions of ACA, accepting funding tied to the law, or assisting federal employees with its implementation. Before issuing the order, Otter vetoed a bill that would have nullified ACA's application in Idaho because he feared it would have prevented the state from legally creating its own insurance exchange.