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Mr. Cannon said even if the judge had upheld the penalty as a tax, it still could have been found unconstitutional because it doesn’t fit under the authorized types of taxes: income, excise or direct taxes.

“Even if they were right it was a tax, it would be an unconstitutional tax,” he said.

Democratic lawmakers tried to portray the ruling as a political decision, rather than a legal one.

Reps. Sander Levin and Pete Stark, two high-ranking Democrats on one of the House committees with jurisdiction over the law, pointedly noted that Judge Hudson was appointed by a Republican president, and accused him of “judicial activism.”

More than 20 lawsuits have been filed challenging the overhaul, and both sides expect the buck ultimately to stop with the U.S. Supreme Court.

On Monday, several congressional Republicans called on the administration to join Virginia Attorney General Kenneth T. Cuccinelli II’s request for an expedited review by the high court. The tax-versus-penalty issue is likely to dog the case all the way.

“The dictionary at times looked like a major impediment to the federal government,” Mr. Cuccinelli said in a press conference call Monday.

Republicans, who in January will take over the House and see their numbers in the Senate minority rise, campaigned this November on a pledge to repeal at least part of the Democrats’ law.

“The nonpartisan Congressional Research Service called requiring people to buy a good or service or be penalized a ‘novel issue,’ and now a federal court has ruled it unconstitutional,” said Sen. Charles E. Grassley, Iowa Republican. “The ruling is likely to be appealed, but it’s a clear signal that the constitutionality of the law, which was moved through Congress with a lot of controversy and partisanship, isn’t as certain as its supporters have argued.”

Seth McLaughlin contributed to this article.