The most politically charged feature of the Medicare legislation passed by Congress — its attempt to make the federal Medicare program compete with private managed-care plans — is also the least likely to come to fruition on the seven-year schedule set in the bill, according to health policy experts. ...

Many people enrolled in Medicare fear that they will end up with less generous benefits in a privately run program, said Len Nichols, a vice president of the Center for Studying Health System Change, a nonprofit research group in Washington. Nor do hospitals and doctors like the idea of health insurers pushing down fees to make a profit for themselves, and health plans have balked at previous projects that threatened to squeeze their profit margins.

In addition, many privately run Medicare plans, known as Medicare H.M.O.'s, withdrew from many areas of the country when government payments lagged, forcing millions of patients to scramble to obtain new coverage. ...