President Obama — a onetime community organizer — is a big supporter, and he has called public service “a central cause” of his administration. In March, the White House said that “AmeriCorps may be one of America’s best assets,” transforming communities every day.

Rick Christman of Lexington, Ky., a former member of the board of AmeriCorps’ parent organization, the Corporation for National and Community Service, said no one expected that Vista workers would be subject to penalties because their health coverage was inadequate.

“It’s unfortunate,” Mr. Christman said.

Samantha Jo Warfield, a spokeswoman for the agency, said AmeriCorps members had several options: They can keep their current coverage; they can shop for coverage on the new insurance exchanges, including HealthCare.gov; and if they are under 26, they may be able to stay on their parents’ insurance. In addition, some might be eligible for Medicaid, the federal-state program for low-income people, which is being expanded in about half the states.

AmeriCorps members say their existing coverage, which pays for doctors’ services, hospital care and prescription drugs, meets most of their needs. However, according to the members’ handbook, “AmeriCorps does not provide benefits for any diagnosis that is considered a pre-existing condition,” and the coverage for preventive care appears to be less than the law requires.

Sarah L. Sklaw, a 22-year-old Vista member from New York City, said: “I really support the Affordable Care Act, and I don’t want to be a naysayer. But it was surprising and frustrating to be told that our health coverage would not meet the law’s standards, especially because the Corporation for National and Community Service told us at orientation in August that we did not need to worry about the issue.”

Other AmeriCorps members said they occasionally needed more extensive coverage to pay for treatment of pre-existing conditions or injuries requiring specialty care, and they noted that some members did dangerous work, such as fighting wildfires or building trails on steep mountains.

The AmeriCorps health plan is available at no cost to members of Vista and the National Civilian Community Corps, a residential program for young men and women, who help build homes, tutor children and provide disaster relief and other services.

Vista officials said that some members might qualify for exemptions from the individual mandate penalty because of their low incomes. Vista members receive allowances to cover the cost of food, housing and other basic necessities. But the amounts are low because members are expected to live at approximately the same economic level as those they serve.