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“When you are changing the relation of the individual to the government in this, what we can stipulate is, I think, a unique way, do you not have a heavy burden of justification to show authorization under the Constitution?” Justice Kennedy asked.

The comment was a welcome one for the challengers.

Randy E. Barnett, a Georgetown University law professor who helped write the plaintiffs’ briefs, said they have argued all along that the individual mandate is an unprecedented overstep of federal power and that is why the justices can’t rely on past cases to make their decision.

“We have said all along that those cases do not control this case because this is fundamentally different,” Mr. Barnett said. “The whole tenor of the discussion really from all the justices was that those cases don’t tell you what to do here.”

Jonathan Turley, a George Washington University law professor, said the justices also may have stayed away from discussing past cases because the jurisprudence over the Commerce Clause is “something of a muddle.”

Over a long period of expanding the power of the Commerce Clause in case after case, the court periodically has stepped in and dialed back federal powers, he said.

That could be a positive sign for the challengers, he said, because the justices seemed to stay away from the weeds and instead pressed the administration on the very points it wanted to avoid.

“The court began where it should have begun with limiting principles,” he said. “And what was remarkable is that the administration seemed almost unprepared or unwilling to answer those questions with any clarity.”