“It will be essentially health reform for blue states,” said John Holahan, a health policy fellow at the Urban Institute, a research group.

On Tuesday, the United States Court of Appeals for the District of Columbia Circuit ruled that the law didn’t allow the federal government to offer financial assistance to people buying insurance in states not running their own insurance marketplaces. That ruling is a long way from final — a second federal court, considering the same issue, ruled to leave the subsidies untouched. If the Washington court’s logic was endorsed by the Supreme Court, it could mean that millions of residents in 36 states would lose access to insurance through the A.C.A.

But even if the current cases fizzle, as some legal analysts expect, there is an existing red-state-blue-state fissure in insurance expansion under the Affordable Care Act.

Two years ago, the Supreme Court ruled that another key provision of the law, one that expanded the Medicaid insurance program to all low-income Americans around the country, would be optional for states. Now that the expansion is underway, existing disparities in rates of insurance have widened among the states. About half the states decided not to expand Medicaid. Recent surveys show that those states that did expand it have significantly reduced their uninsured population. An Urban Institute survey found that the uninsurance rate among adults under 65 had declined by 6.1 percentage points in states that expanded, compared with only 1.7 percentage points in those that didn’t.