The United States is the only western industrialized nation that fails to provide universal coverage and the only nation where health care for the majority of the population is financed by for-profit, minimally regulated private insurance companies. These arrangements leave one-sixth of the population uninsured at any given time, and they leave others at risk of losing insurance as a result of normal life course events. Political theorists of the welfare state usually attribute the failure of national health insurance in the United States to broader forces of American political development, but they ignore the distinctive character of the health care financing arrangements that do exist. Medical sociologists emphasize the way that physicians parlayed their professional expertise into legal, institutional, and economic power but not the way this power was asserted in the political arena. This paper proposes a theory of stakeholder mobilization as the primary obstacle to national health insurance. The evidence supports the argument that powerful stakeholder groups, first the American Medical Association, then organizations of insurance companies and employer groups, have been able to defeat every effort to enact national health insurance across an entire century because they had superior resources and an organizational structure that closely mirrored the federated arrangements of the American state. The exception occurred when the AFL-CIO, with its national leadership, state federations and union locals, mobilized on behalf of Medicare.