When insurance companies lobby and buy advertising, the general assumption is that they are doing so in their own interest. There may be a public interest as well, but its a secondary motive (at best) for the corporation. When a union supports a candidate with money and volunteers, we can also assume it is doing so in its own interest. While there may be a public interest, it is not the motive for the union's action. When a trade organization, like the National Association for Manufacturers, lobbies and buy advertising, it's most like that they are doing so in their own interest. The public interest, if any, is secondary.
And then there's the American Medical Association. The AMA is a trade association. Even among doctors it represents only a fraction of the population. According to Carol Paris, M.D., the AMA represents less than a third of American doctors. Of those it represents, half are retired.
When the AMA speaks for "America's doctors," it is--like any other lobbying group--looking out for its own interests. At best, those interests correspond to a fraction of the population of doctors. At worst, it's just looking out for itself. If (and I admit it's an ENORMOUS if) a single-payer system, or anything else, could significantly improve patient care at less cost, the AMA and its members would be the losers.
Whenever someone says they speak for "the doctors," or "the lawyers," or "the workers," or the businessmen," or "the students," ask for names, check identifications, and figure out who really benefits or loses, and by how much.