In the late 1950s, liberal Democrats were pushing for old age health insurance as an addition to Social Security. The original push had been for national health insurance in the form of the Wagner-Murray-Dingell Bill, which would have effectively established the equivalent of the National Health Service in the United States. However, the proposal lacked support from Republicans and Southern Democrats. Thus, the liberal Democrats decided to act more incrementally, choosing to seek aid for the elderly first. Thus, the idea of Medicare was born. Rep. Wilbur Mills (D-Ark.), chair of the Ways and Means Committee, opposed the idea, as did powerful wheeler-dealer Sen. Robert Kerr (D-Okla.). However, instead of outright opposition, they proposed a substitute measure. This measure would provide federal grants to states that established their own old age health insurance system. Liberals tried to insert the Medicare Bill in a Senate vote, but they lost 44-51. The GOP tried to insert its own proposal which would provide subsidies for purchasing health insurance, but this was soundly defeated on party lines. The problem was, far from all states adopted it, and it was regarded by advocates of Medicare as partially successful at best and a failure at worst. Even from its very inception, prominent politicians considered it weak sauce. JFK called the measure insufficient and Richard Nixon concurred in this judgment during the 1960 presidential campaign. I get the feeling that the policy was not given enough time to fully work, but history sometimes moves faster than a few years.
At the time of passage, however, the law was highly popular, even earning the praise of Ronald Reagan if not the enthusiastic endorsement of the American Medical Association (AMA), at the time a staunchly conservative organization. Its opponents were quite few in Congress, with the only Senators voting against being Barry Goldwater and Strom Thurmond. Granted, many liberals simply thought this measure better than nothing. Liberals tried to pass Medicare again in 1964, but it narrowly failed a Senate vote. However, the 1964 election resulted in the Great Society Congress, and Mills (Kerr was dead) ended up supporting Medicare, albeit with some changes to win people over. The 1965 Social Security Amendments also included the creation of Medicaid, health insurance for the poor.
McKee, G. (24 February 2016). Prescription for Success. The Miller Center.
Moore, J.D. & Smith, D.G. Legislating Medicaid: Considering Medicaid and Its Origins
Social Security: Chapter 4: The Fourth Round
Retrieved from https://www.ssa.gov/history/corningchap4.html