Wasn not the original plan with the Medicare? It was supposed to be expanded to a national health care program.
I also disagree with the opinion that physicians hold the key to a national health care plan. It is the insurance companies and employers, stupid. National health care system is generally favourable to physician's occupational interests for the same reason as keynesianism being favoured by defence contractors: it signioficantly boosts the demand for the product and the revenues of the providers. The introduction of Medicare boosted physicians' income quite significantly. Moreover, a government-sponsored program is less likely to micromanage health care delivery than a profit conscious HMO, and thus it allowes for a greater occupational autonomy.
The main reason why US physicians have been generally happy with the current private insuranca company run system is that until recently it gave substantial occupational autonomy to the provider, while passing the cost on coporate premium payers, who in turn passed them on their employees. However, as employers are anxious to cut health insurance cost, the pressure on providers will mount.
Another issue is why US employers did not "socialize" the cost of health insurance as employers elsewhere did. The answer to this question is: power. Health benefits are a weapon to keep the unions at bay and workers in line, whereas a universal health system would take the power to deny those benefits from the bosses' hands.
In sum, the national health system is unlikely in this country not because doctors oppose it, but because corporate bosses do not like it.
wojtek