The Florida judge wrote that "the power that the individual mandate seeks to harness is simply without prior precedent."
He said the plaintiffs—20 states led by Florida's attorney general—had "most definitely stated a plausible claim" in calling the new health-care law's mandate unconstitutional.
That contention is at the center of a nationwide legal challenge to the administration's signature domestic-policy legislation, which is also a top issue in the midterm elections.
Plaintiffs had also argued that the law's provision expanding Medicaid to 16 million Americans was unconstitutional because it would impose costs that the states couldn't afford. Judge Vinson expressed skepticism about this argument but allowed the claim to go forward.
Judge Vinson cautioned, however, that he hadn't made up his mind on the merits of the case, and handed the Obama administration some smaller victories, dismissing the plaintiffs' four other contentions.
"While we are disappointed that the court did not dismiss the entire case, we welcome the judge's decision to reject most of the claims," said Tracy Schmaler, a spokeswoman for the Department of Justice. "We remain confident that the law ultimately will be upheld."
The case is expected to wind up before the U.S. Supreme Court. Another federal judge in Michigan has already ruled in the administration's favor in a separate case.
The Florida case, brought by a group of 18 Republican attorneys general and governors and two Democratic attorneys general, is one of about 20 suits challenging the overhaul, and the insurance mandate is the chief target. Plaintiffs argue that requiring people to carry insurance or pay a fine exceeds Congress's powers under the Constitution to regulate economic activity, because they say the decision to go without insurance isn't an activity.
The Obama administration says that nearly all Americans get medical care through a doctor or hospital, and that requiring them to carry insurance simply regulates how they pay for services they will inevitably demand when they get sick.
The latest development raises a puzzling question for both sides: Would the whole law get struck down if a federal court finds the individual mandate in violation of the Constitution?
The plaintiffs argue that the entire law should be invalidated. The administration counters that many other pieces of the legislation could stand withoutthe individual mandate, including the expansion of Medicaid.
Democrats who drafted the law didn't include a "severability" clause, which would spell out that remaining parts of the law could stay intact should certain parts be struck down. A Democratic aide who helped draft the law said staff members discussed such a clause but decided it wasn't necessary.
Many legal scholars believe the administration is likely to win the core case, but they don't rule out a plaintiffs' victory. They say it is surprising that Democrats didn't include a severability clause just in case.
During the law's drafting, Democrats argued that a requirement to buy coverage was essential to ensure that insurers had a broad mix of healthy and sick enrollees. Otherwise, they said, people could wait until they needed care to get covered, since the law also requires that insurers accept all comers regardless of health status.
"In the absence of the minimum-coverage provision, the incentive for delay would drive the insurance market 'into extinction,' " the administration wrote in June responding to a similar health-law challenge in Virginia, citing congressional testimony by Uwe Reinhardt, a professor at Princeton University.
But in a response to that case filed in September, the administration contended that the entire law shouldn't be struck down, should the court decide the mandate violates the Constitution. It conceded that the law's requirement that insurers supply coverage couldn't stay in place without the mandate.
The administration said the expansion of Medicaid and many other sections—including an unrelated measure to expand federal student-loan funding that got tacked on to the health law—could survive.
"It is far from 'evident' that Congress would have preferred all of these provisions to be invalidated if the minimum coverage were to fall," the administration wrote.
Legal scholars said the government's attempt to preserve pieces of the law could contradict its contention that the individual mandate is an essential part of the overhaul.
"There may be some tension in that part of the government's argument," said Robert Schapiro, professor of law at Emory University.
"If the argument is also that 'Well, if you strike down the individual mandate, you can leave the rest of the law intact,' it's not exactly essential," he said.
Randy E. Barnett, a Georgetown University Law Center professor who opposed the overhaul, said it was a "very tenuous" case for the government to make.
"Two sides of its argument are at war with each other," he said.