More than 50 million Medicaid recipients will soon have to produce birth certificates, passports or other documents to prove that they are United States citizens, and everyone who applies for coverage after June 30 will have to show similar documents under a new federal law.

The requirement is meant to stop the "theft of Medicaid benefits by illegal aliens," in the words of Representative Charlie Norwood, Republican of Georgia, a principal author of the provision, which was signed into law by President Bush on Feb. 8.

In enforcing the new requirement, federal and state officials must take account of passions stirred by weeks of national debate over immigration policy. State officials worry that many blacks, American Indians and other poor people will be unable to come up with the documents needed to prove citizenship. In addition, hospital executives said they were concerned that the law could increase their costs, by reducing the number of patients with insurance.

The new requirement takes effect on July 1. The Congressional Budget Office estimates that it will save the federal government $220 million over five years and $735 million over 10 years.

Estimates of the number of people who will be affected vary widely. The budget office expects that 35,000 people will lose coverage by 2015. Most of them will be illegal immigrants, it said, but some will be citizens unable to produce the necessary documents. Some Medicaid experts put the numbers much higher, saying that millions of citizens could find their health benefits in jeopardy. 

State officials are trying to figure out how to comply. Many said the requirement would result in denying benefits to some poor people who were entitled to Medicaid but could not find the necessary documents.

"This provision is misguided and will serve as a barrier to health care for otherwise eligible United States citizens," said Gov. Chris Gregoire of Washington, a Democrat.

Ms. Gregoire said the provision would cause hardship for many older African-Americans who never received birth certificates and for homeless people who did not have ready access to family records.

Hospitals and nursing homes are expressing concern. "The new requirement will result in fewer people being eligible for Medicaid or enrolling in the program, and that means more uninsured people," said Lynne P. Fagnani, senior vice president of the National Association of Public Hospitals and Health Systems. "They still need care, but are more likely to wait until their condition becomes more severe and more costly to treat."

The new requirement will come as a surprise to most Medicaid recipients. The law said federal officials should inform them "as soon as practicable" after Feb. 8. But the education campaign, to be conducted in concert with states, has yet to begin.

Under the law, the Deficit Reduction Act, states cannot receive federal Medicaid money unless they verify citizenship by checking documents like passports and birth certificates for people who receive or apply for Medicaid.

In a draft letter providing guidance to state officials, the Bush administration says, "An applicant or recipient who does not cooperate with the requirement to present documentary evidence of citizenship may be denied eligibility or terminated" from Medicaid.

The Center on Budget and Policy Priorities, a liberal research and advocacy group, estimates that three million to five million low-income citizens on Medicaid could find their coverage at risk because they do not have birth certificates or passports.

Representative John Lewis, Democrat of Georgia, said: "Many older Americans do not have birth certificates because their parents did not have access to hospitals, and so they were born at home. In the last century, all over the South, because of segregation and racial discrimination, many hospitals would not take minorities."

In Georgia, Medicaid officials began enforcing a similar requirement in January. Dr. Rhonda M. Medows, commissioner of the state's Department of Community Health, said it had not caused serious problems.

In Arizona, the governor's health policy adviser, Anne M. Winter, said the federal requirement would "reduce or delay enrollment for eligible individuals, mostly U.S. citizens." In many cases, Ms. Winter said, "Native Americans — the first Americans — do not have the documents" required to show citizenship. In addition, she said, older Medicaid recipients with Alzheimer's disease or other mental impairments may not understand the requirement and may be unable to retrieve the documents they need.