Credible commitment is central to regimes for climate mitigation policy. In the climate policy literature, it is widely argued that the solution to the credible commitment problem is legislation and delegation of goal-setting to a technical body insulated from political incentives, and the UK’s Committee on Climate Change is in part modelled on this approach. However, drawing on the comparative politics literature, this paper argues that the focus on legislation and delegation as the solution to the credible commitment problem is too narrow. Seen within the context of comparative political institutions, it is a response that fits the political logic in countries with majoritarian electoral systems. By contrast, in countries with electoral systems based on proportional representation, while legislation plays a role, an important element in the creation of credible commitment comes in the form of negotiated long-term agreements between political parties. This contrast is explored through a comparison between the Climate Change Act and associated Committee on Climate Change in the UK on the one hand, and a series of Energy and Climate Agreements in Denmark over the 2010s. Both approaches appear to have worked to date. However, while negotiated long-term agreements typically have an internal process for managing conflicts that inevitably arise after the respective mechanisms have been put in place, disputes arising following legislation and delegation must be resolved within the more informal processes of intra-party politics. Mechanisms of accountability also differ between the two approaches.