Jonathan E. Chapman, the executive director of the Louisiana Primary Care Association, which represents more than two dozen community health centers, described the situation in his state this way: “If the breadwinner in a family of four works full time at a job that pays $14 an hour and the family has no other income, he or she will be eligible for insurance subsidies. But if they make $10 an hour, they will not be eligible for anything.”

Bruce Lesley, the president of First Focus, a child advocacy group, said: “In states that do not expand Medicaid, some of the neediest people will not get coverage. But people who are just above the poverty line or in the middle class can get subsidized coverage. People will be denied assistance because they don’t make enough money. Trying to explain that will be a nightmare.”

The subsidies, for the purchase of private insurance, will vary with income and are expected to average more than $5,000 a year in 2014 for each person who qualifies.

Evan S. Dillard, the chief executive of Forrest General Hospital in Hattiesburg, Miss., said the eligibility rules would be “very confusing to working poor individuals in this, the poorest state in the country.”

Starting in January, most Americans will be required to have health insurance and will be subject to tax penalties if they go without coverage. However, the penalties will not apply to low-income people denied access to Medicaid because they live in states that chose not to expand eligibility.

Deborah H. Tucker, the chief executive of Whatley Health Services, a community health center in Tuscaloosa, Ala., said it was wonderful that many uninsured people would gain coverage, but “tragic that some of the most vulnerable, lowest-income people” would be excluded.

Ms. Tucker said her clinics cared for nearly 30,000 patients a year, including 16,000 who were uninsured. More than 75 percent of the uninsured patients have incomes below the poverty level and are unlikely to qualify for Medicaid or subsidies, she said.