Political parties are important actors in domestic climate politics. What drives variation in parties’ climate policy preferences? To contribute to a growing literature on the party politics of climate change, we focus on the roles of public opinion, party competition, and parties’ traditional policy preferences in shaping parties’ climate policy preferences in Denmark and Ireland. In case studies that draw on in-depth interviews with policy practitioners, we show how parties respond to public opinion, accommodate issue-owners, and are powerfully constrained and enabled by their existing preferences. These mechanisms also help to explain different responses on climate policy across the left-right spectrum. Competition between mainstream parties is particularly powerful, but can constrain as much as it enables ‘greener’ climate policy preferences. While climate change may be a distinctive problem, the party politics of climate change features similar incentives and constraints as other domains.