Why have states legalised international norms promoting domestic democracy in some regions of the world? This issue poses a difficult puzzle because standard assumptions about state preferences for sovereignty make the creation of strong, binding international rules on democracy unlikely. We identify four possible answers: the interests of powerful states, common government interests in domestic policy lock-in, the absence of fears that powerful states will use the rules to intervene unilaterally in domestic affairs, and the robustness of pre-existing norms. We explore our argument by applying it to the Organization of American States (OAS), a fairly unlikely organisation for the strong legalisation of international rules. Our findings suggest that legalisation of democracy is quite difficult to achieve. State interests in locking in democratic benefits and state power – even hegemonic power – are necessary but insufficient. An important set of new democracies attempted legalisation in the 1950s, yet failed. The United States, as strong a regional hegemon as ever, attempted further legalisation in 2005, yet failed. Motives and power must be accompanied by low fears of unilateral intervention and high levels of norm robustness in order to produce results.