The Bush administration said Thursday that it would exempt millions of the most vulnerable Medicaid recipients from a new law that requires them to prove they are United States citizens by showing birth certificates, passports or other documents. 

The action was apparently intended to pre-empt a ruling by a federal judge who is scheduled to hold a hearing on Friday on a lawsuit challenging the new requirement, which took effect on July 1.

Dr. Mark B. McClellan, administrator of the Centers for Medicare and Medicaid Services, said that more than 8 million of the 55 million Medicaid recipients would be "exempt from the new documentation requirement" because they had established their citizenship when they applied for Medicare or Supplemental Security Income. 

Medicaid, financed jointly by the federal government and the states, provides health insurance for low-income people, including many in nursing homes. Medicare provides health insurance for people who are 65 and older or disabled. Supplemental Security Income is a cash assistance program for people with very low incomes who are elderly, blind or disabled. About six million people receive Medicare and Medicaid. In most states, people receiving Supplemental Security Income are entitled to Medicaid.

Dr. McClellan said the exemption would apply, for example, to "people with mental retardation who have never worked and to many nursing home residents." Critics of the new law had said it would be difficult for many people with mental retardation, Alzheimer's disease and other mental impairments to produce the documents needed to comply.

Under the law, anyone who has Medicaid coverage or applies for it must present "documentary evidence of citizenship." Previously, more than 40 states had accepted the applicants' written statements as proof of citizenship unless the claims seemed questionable. "Self-attestation of citizenship and identity is no longer an acceptable practice," the administration said in a rule issued on Thursday evening.

The new documentation requirement is part of the Deficit Reduction Act, signed by President Bush on Feb. 8. It 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.

In an unusual preamble to the new rule, the Bush administration said it believed that Congress had intended to exempt Medicaid beneficiaries who were also receiving Medicare or Supplemental Security Income.

The law says the documentation requirement "shall not apply to an alien who is eligible for medical assistance" if the person is also enrolled in one of the other two programs. 

The administration said this was "clearly a drafting error." Congress intended an exemption for citizens, "but accidentally used the term 'alien,' " the preamble says.

A literal reading of the statute would lead to an absurd result — "an exemption applying only to aliens who declare themselves citizens" — and would be of no help to the intended beneficiaries, those citizens who would have the most difficulty proving their citizenship, the administration said. The agency said it was merely correcting "a scrivener's error," one of several it found.

John E. Stone, a spokesman for Representative Norwood, said the final rule "appears to provide an appropriate degree of flexibility" to Medicaid beneficiaries and to states.

Ronald F. Pollack, the executive director of Families U.S.A., a consumer group working with plaintiffs in the court case, said: "The exemption of seniors and people with severe disabilities from the citizenship verification requirement is a commendable development. 

But many other people who need health care the most and can't come up with the required documentation — such as foster children and homeless people — may still lose Medicaid coverage and join the ranks of the uninsured. This should be corrected."

Dr. McClellan said that states could help establish a person's identity and citizenship by using records of state agencies that administer food stamps, child support and child protective services, as well as the agencies that issue driver's licenses.