WASHINGTON  As the health care debate heats up, the American Medical Association is letting Congress know that it will oppose creation of a government-sponsored insurance plan, which President Obama and many other Democrats see as an essential element of legislation to remake the health care system.

The opposition, which comes as Mr. Obama prepares to address the powerful doctors’ group on Monday in Chicago, could be a major hurdle for advocates of a public insurance plan. The A.M.A., with about 250,000 members, is America’s largest physician organization.

While committed to the goal of affordable health insurance for all, the association had said in a general statement of principles that health services should be “provided through private markets, as they are currently.” It is now reacting, for the first time, to specific legislative proposals being drafted by Congress.

In the presidential campaign last year and in a letter to Congress last week, Mr. Obama called for a new “public health insurance option,” which he said would compete with private insurers and keep them honest.