IN A VOTE that both reflects and could accelerate the extraordinary changes that have swept the U.S. health care system in recent years, the American Medical Association has decided to help doctors unionize. The importance for now is more symbolic than substantive. Unless the law is changed, only a minority of doctors would currently be eligible to join a union, and many of those don't want to.

The apparently close vote in the organization's house of delegates is nonetheless an indicator of how aggrieved many doctors feel in a world in which buyers of health care have increasingly consolidated and otherwise bulked up to improve their bargaining power and hold down costs. The various forms of managed care are the main expression of this trend, in which doctors have lost some of their traditional control not just over fees but over the way they practice medicine. Hospitals and other providers of care have responded to the trend by doing some consolidating and other body-building of their own. In some communities, doctors have created health care plans that they themselves continue to control. Unionization would be a variation on the theme.

Under current law, only doctors who are employees of health care organizations could join a union. Legislation pending in Congress would grant an antitrust exemption to self-employed physicians -- still far and away the majority -- whereby they, too, could in effect bargain collectively as to fees and the question of who decides what care will be paid for. But the bill has opponents. Federal Trade Commission Chairman Robert Pitofsky told a House committee the other day that the FTC fears it would increase costs. The commission "continues to believe [in the value of] competition among health care providers," he said.

The AMA said its union would be different from others in that it wouldn't strike and that its primary goals weren't economic. "We are looking for a vehicle that will allow us to carry out the covenant we have with our patients," its president said after the vote. If that happened to coincide with an increase in physicians' incomes, that would be a secondary result, AMA officials said.

Still, the transformation of the health care system has put the AMA in strange political territory. On legislative issues, its traditional alignment has been with the Republicans. On the current question of regulating managed care, however, it finds itself on the side of Democrats who want to curb the power of the cost-controllers to refuse to pay for care; it is opposed to the business and insurance groups whose interest is to hold down costs. The question of cost has altered health care politics in the same way it has altered everything else about the system.