PENSACOLA, Fla.  A federal judge asserted on Thursday that it would be “a giant leap” for the Supreme Court to accept the Obama administration’s defense of a central provision of the new health care law, suggesting he may become the second judge to strike it down as unconstitutional.

In a three-hour hearing, the judge, Roger Vinson of Federal District Court, said the law’s requirement that most Americans obtain insurance, a provision that takes effect in 2014, would constitute “a giant expansion” of the court’s traditional application of the Commerce Clause of the Constitution.

“People have always exercised the freedom to choose whether to buy or not buy a commercial product,” the judge said, noting that he had been uninsured and paid out of pocket when his first son was born.

The hearing came on dueling requests for summary judgment in a lawsuit brought by governors and attorneys general from 20 states, all but one of whom are Republicans. Because of the plaintiffs’ prominence, the cases carry the most political weight of the roughly two dozen court challenges to the sweeping law.