The Sanders plan — which has no near-term chance of advancing with Republicans in power — would change that. Most crucially, it would expand the program so that every American would eventually get insurance from Medicare instead of private companies or other public programs. The employer health insurance system, the Affordable Care Act exchanges and most of Medicaid would be eliminated.

Mr. Sanders says that the transition to the Medicare program would achieve several goals: It would ensure universal coverage, it would improve the affordability of health care for many Americans and it would save the country money.

Whether it can achieve that third goal depends a great deal on how the new Medicare-for-all system would be managed, and the Sanders plan leaves a lot of those details unclear for now. Covering everyone would not, by itself, make the health care system in the United States as inexpensive as those of other nations with universal health care systems. That sort of cost control would require its own set of policies and difficult choices.

In his news conference introducing the bill on Wednesday, Mr. Sanders focused his ire on insurance and drug companies, two profitable sectors of the health care system. But insurance and drug company profits don’t make up the bulk of America’s health care spending. Single-payer advocates argue that the simplicity of a single, government payer would reduce paperwork and office staff, and that is almost certainly true. But most spending in the health care system is on medical care from doctors and hospitals — and squeezing savings there may be harder.