By conventional indicators—such as national wealth, the longevity of regime institutions, and an elaborate system of checks and balances designed to guard against unwarranted concentrations of power—U.S. democracy should be virtually immune from autocratic threats. Nevertheless, over the past decade U.S. democracy has encountered unprecedented challenges due to the dramatic strengthening of authoritarian political actors and their ready access to national governing institutions. Although democracy has not broken down, it has been severely degraded and unsettled, and cannot credibly be considered “consolidated.” Pockets of democratic self-defense are surely present in the courts, the media, civil society, and the electorate, but in a context of hyper-polarized two-party competition, all of these are sites of regime contestation. This paper analyzes how and why polarized two-party competition makes it especially difficult to activate strategies of democratic self-defense, largely because (1) neutral guard-rail institutions become heavily politicized sites for partisan advantage or retribution, and (2) dynamics of partisan bandwagoning and opportunism insulate political leaders from accountability for violating democratic norms and procedures. Special attention will be given to the strategies adopted by democratic forces to work around these constraints and keep democratic spaces open, even when two-party competition is de facto transformed into a regime cleavage.