WASHINGTON » President Barack Obama has made health care his top priority. He says the cost of Medicare and Medicaid is “;the biggest threat”; to the nation's fiscal future. But to the puzzlement of Congress and health care experts around the country, Obama has not named anyone to lead the agency that runs the two giant programs.

The agency, the Centers for Medicare and Medicaid Services, is the largest buyer of health care in the United States. Its programs are at the heart of efforts to overhaul the health care system. If it had an administrator, that person would be working with Congress on legislation and could be preparing the agency for a new, expanded role.

“;The vacancy stands out like a sore thumb,”; said Dr. Denis A. Cortese, president of the Mayo Clinic, often cited by the White House as a health care model.

“;In effect,”; Cortese said, “;Medicare is the nation's largest insurance company. The president and Congress function as the board of directors.

“;Under a strong administrator, it could take the lead in making major changes in the health care delivery system, so we'd get better outcomes and better service at lower cost.”;

The agency provides health insurance to 98 million people, pays 1.2 billion claims a year and has an annual budget of more than $700 billion. It has a pervasive influence on medical care, regulating hospitals, doctors, health plans, laboratories and almost every other type of health care provider. When Medicare decides to cover a new treatment or adopts a new payment policy, private insurers often follow its lead.

Trying to remake the health care system without a Medicare administrator is like fighting a war without a general.

“;You need a general,”; said Sen. Jay Rockefeller, D-W.Va., chairman of the health subcommittee of the Finance Committee. Of the job vacancy, Rockefeller said: “;It's a big problem. I can't explain it.”;

Administration officials said they were searching for someone with the right mix of managerial experience and clinical expertise.

The agency has not had a regular Senate-confirmed administrator since October 2006, when Dr. Mark B. McClellan stepped down. Its chief operating officer, Charlene M. Frizzera, has been the acting administrator since January.

Dr. John C. Lewin, chief executive of the American College of Cardiology, said that, in the absence of an administrator, many decisions were being made by “;a beleaguered bureaucracy.”;