But federal officials have not set forth a strategy to expand access to care with enrollment, and in many states Medicaid payment rates for primary care services, like routine office visits and the management of chronic illnesses, will plunge back to 2012 levels, widely seen as inadequate.

For the last two years, the federal government has required state Medicaid agencies to pay at least as much as Medicare pays for primary care services. Family doctors, internists and pediatricians have thus received Medicare-level payments for primary care, with the federal government making up the difference in costs.

The impending cuts are larger in states like California that have the widest gaps between Medicaid and Medicare rates.

A survey by the Ohio State Medical Association found that some Ohio doctors began accepting Medicaid patients because of the rate increase in 2013. Ohio doctors who were already participating in the program said they had accepted more Medicaid patients after the rate increase. And almost 40 percent of Ohio doctors indicated that they planned to accept fewer Medicaid patients when the extra payments lapsed.

Under federal law, Medicaid rates must be “sufficient to enlist enough providers” so that beneficiaries have at least as much access to care as the general population in their geographic area. In practice, doctors say, this standard is murky.

The Obama administration told the Supreme Court last month that health care providers had no legal right to enforce the “equal access” requirement in court. This section of the Medicaid law provides guidance to federal and state officials in setting Medicaid rates, but does not allow health care providers to sue state officials to enforce it, said Donald B. Verrilli Jr., the solicitor general of the United States.

The case, Armstrong v. Exceptional Child Center, was filed against Richard Armstrong, director of the Idaho Department of Health and Welfare, by five providers of residential habilitation services to children with disabilities. They argued that Idaho’s payment rates fell below federal standards, and they sued to enforce federal law, invoking the supremacy clause of the Constitution, which makes federal law “the supreme law of the land.”