Over the past twenty years, Medicare has been transformed from a single-payer insurer into a hybrid of complementary public and private insurance arrangements. Despite creating ongoing controversy, these changes have resulted in an ironic and largely overlooked strategic potential: Medicare's evolving hybrid form makes it the most promising vehicle for overcoming the historical obstacles to universal health insurance in the United States. To make this surprising case, we first explore the distinctive political dynamics of programs that, like today's Medicare, are hybrids of public and private arrangements. We then consider how these political dynamics might circumvent past barriers to universal health insurance. Finally, we discuss the strengths and weaknesses of alternative pathways through which Medicare could be expanded to promote health security.