This essay explores the possibilities and limits of partisan influence on public policy in democratic nations. It will be pointed out, that differences in party composition of government, in general, matter in public policy in constitutional democracy. However, the extent to which parties influence public policy is to a significant extent contingent upon the type of democracy and countermajoritarian institutional constraints of central state government. Large partisan effects typify majoritarian democracies and states, in which the legislature and the executive are ‘sovereign’. More complex and more difficult to identify is the partisan influence on public policy in consensus democracies and in states, in which the political-institutional circumstances allow for co-governance of the opposition party. Narrowly circumscribed is the room to manoeuvre available to incumbent parties above all in political systems which have been marked by countermajoritarian institutional pluralism or institutional ‘semi-sovereignty’. The article suggests, that it would be valuable if direct effects and interaction effects of the party composition of government and state structures featured more prominently in future research on comparative public policy.