Abstract The Nuclear Nonproliferation Treaty (NPT) forms the core of the most prominent multilateral security governance regime of the liberal international order (LIO). Despite being a widely supported and remarkably successful security regime to prohibit the spread of nuclear weapons, NPT politics have become increasingly polarized. In this article, I analyze contestation and self-undermining dynamics in the nuclear regime. The NPT is a strongly institutionalized but weakly liberal regime. However, it is unevenly institutionalized. Patterns of contestation in the regime derive from these characteristics. The basic source of contestation is the fundamental inequality between the nuclear “haves” and “have-nots.” Specifically, dissatisfied states seek to strengthen the rules and obligations in the less institutionalized domain. I argue that when efforts at order-consistent reform fail, states shift to a more radical strategy of liberal counter-institutionalization to pursue their goal of equality. Nonnuclear states have long been dissatisfied with the uneven distribution of the burdens and benefits of the regime, and especially the nuclear-armed states’ failure to pursue their obligation of disarmament. Most of the contestation in the NPT over the decades has been order-consistent and pursued “liberal reform.” However, when proponents of nuclear disarmament failed to achieve meaningful progress through the NPT, they shifted to a liberal counter-institutionalization strategy and adopted an entirely new legal instrument, the 2017 Nuclear Prohibition Treaty, to pursue disarmament. This was a direct—but liberal—challenge to the existing suborder. This treaty, which prohibits nuclear weapons for all, legalizes the NPT obligation to pursue disarmament.