Although President Kennedy had not been able to get majority support for a national health insurance program while he was alive, after he was assassinated the programs he had been espousing took on new life. Lyndon Johnson ran for President in 1964 promising to implement Kennedy's plans, and won by a landslide, convincing many opponents that Medicare had overwhelming public support.
There was still a lot of debate, and intense opposition from the American Medical Association (AMA). Some opponents were concerned that covering hospital costs would only take care of about 25% of the medical costs of the elderly, and others were concerned about providing more coverage for the poor elderly. Another concern was the need to get the support of Ways and Means chairman Wilbur Mills, who had co-sponsored the Kerr-Mills program for the poorest elderly, and the AMA, which had floated an a voluntary program called "Eldercare" as an alternative to Medicare.
To counter the opposition in order to get the bill passed, all these concepts were incorporated into the legislation, and the 1965 amendments to the Social Security Act established the Medicare and Medicaid programs. Medicare Part A covered hospital costs, Part B was a voluntary program partially financed by premiums which covered physician and other out-of-hospital costs, and Kerr-Mills was converted to a program called 'Medicaid' to cover the indigent elderly.