The National Federation of Independent Business is joining a federal lawsuit filed in Florida by 20 state attorneys general and governors against the health care overhaul.
The nation's most influential small business lobby is joining a court challenge that argues that Americans cannot be required under the Constitution to obtain insurance coverage.
All but one of the state attorneys general are Republicans, and the case coincides with an election year.
With 350,000 members, the NFIB boasts a far-reaching network of local activists.
One of its pillars of the new health care law is the requirement, effective in 2014, that most Americans carry health insurance.
The Obama administration argues that the coverage requirements rest on a solid constitutional foundation: the power of Congress to regulate interstate commerce.