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The ruling is the second one to go against the law, while two other federal judges have upheld it. Still other judges have shot down challenges on technical grounds, though they never decided the merits of the case.

Ever since Mr. Obama signed the health care legislation in March, it has been the dominant issue in American politics, with Republicans campaigning on opposition to the law on their way to winning control of the House and gaining seats in the Senate in last year’s elections.

Already, the House has passed a bill to repeal the entire law, and Senate Republicans have vowed to try to force a vote on the measure in their chamber.

Senate Republicans’ campaign committee also fired off attacks on Democrats facing difficult re-election bids in 2012 who defended the health care law as constitutional.

Democrats, though, say Americans have already become attached to some of the early-acting benefits in the law, such as allowing adult children to remain on their parents’ insurance until age 26 and requiring insurance companies to cover pre-existing conditions.

“Political opponents of the Affordable Care Act are intent on doing in the courts what they could not do in Congress, by rolling back these important protections,” said Senate Judiciary Committee Chairman Patrick J. Leahy, Vermont Democrat.

His committee will hold a hearing Wednesday looking at the constitutional basis for the health care law.

The law took a tortuous path to passage last year, surviving purely based on Democrats’ at-the-time overwhelming majorities in both the House and Senate.

The measure that eventually became law was written to survive the Senate in December 2009, but wasn’t designed to pass the entire Congress. But Sen. Scott Brown’s surprise election victory in January 2010 left Democrats without a filibuster-proof majority, which meant the House had to accept the Senate’s version.

Judge Vinson exposed many of those flaws of that process in his ruling — most notably the lack of a severability clause, which existed in some other versions, but not in the Senate’s final bill. Severability would have allowed some parts of the law to remain in place even if the individual mandate fell.

In his ruling, the judge did uphold the federal government’s argument for rewriting Medicaid, the federal-state health care program for the poor. The states had argued that the new burdens the federal government put on state Medicaid programs was unfair, but Judge Vinson said states always have the option — however unpalatable — of opting out of the program.

But he ruled in favor of states who said the individual mandate went beyond Congress‘ authority as granted by the Constitution.

The case was brought by Florida and 19 other states last year, and just two weeks ago, six other states joined the suit, bringing the total to 26. Virginia Attorney General Kenneth T. Cuccinelli II filed his own lawsuit based on Virginia law, and late last year federal Judge Henry E. Hudson upheld Mr. Cuccinelli’s challenge.