U.S. District Judge Roger Vinson, with this ruling set up a clash over whether the Obama administration still has the authority to carry out the law designed to expand insurance to 32 million Americans.
David Rivkin, an attorney for the plaintiffs, said the ruling meant the 26 states challenging the law must halt implementation of pieces that apply to states and certain small businesses represented by plaintiffs.
But the Obama administration said it has no to plans to halt implementation of the law. Already, it has mailed rebate checks to seniors with high prescription drug costs, helped set up insurance pools for people with pre-existing medical conditions and required insurers to allow children to stay on their parents' insurance policies until they reach age 26.
"We will continue to operate as we have previously," a senior administration official said.
In a pre-emptive move, the Justice Department, which represents the administration, is considering whether to seek a stay while its appeal against the decision is pending, spokeswoman Tracy Schmaler said.
The legal morass is the biggest blow yet to the law since President Barack Obama signed it in March. Most of the plaintiffs—governors and attorneys general in 26 states—are Republicans seeking to knock down Mr. Obama's signature legislative achievement.
The ruling by Judge Vinson, a Republican appointee in Pensacola, Fla., is the second of four to find that at least part of the law violates the Constitution's Commerce Clause by requiring citizens to carry insurance or pay a fee. But in asserting that the whole law is unconstitutional, it went much further than an earlier ruling in a Virginia case.