While current research shows that the government dominates the policy agenda in parliamentary democracies, little is known about the role of the opposition in challenging this dominance. Taking a closer look at the parliamentary policy-making process, we examine whether opposition support for partisan control of committee chairmanship makes challenges to government bills through amendment proposals more or less likely. By analysing about 7400 government bills from three parliamentary democracies over 35 years, our results show that, under opposition chairmanship, a high likelihood of opposition support fosters amendment proposals, but, under coalition partner chairmanship, the likelihood of government bills being challenged only increases when the likelihood of opposition support is low. This suggests that a unified opposition not only makes challenges to the government's agenda more likely but also conditions how coalition partners manage collective governance.
