Many of the models we have considered in previous chapters focus implicitly on the construction of coalitions in support of particular policies. In models of electoral competition, for example, parties or candidates seek the support of majority coalitions among the electorate. In contrast, in models of veto players, an agenda setter endeavors to assemble a unanimity coalition among political actors with the power to block change from the status quo.

In this chapter, the focus on coalitions is explicit. We consider coalition formation both in small groups, such as legislatures and committees, and in polities. Regardless of the setting, there is typically a privileged actor—agenda setter, incumbent, and so forth—who has the first opportunity to form a winning coalition. The models differ with regard to what occurs if coalition formation is unsuccessful: an exogenous default policy is implemented; agenda-setting power passes to another actor; or a challenger (to the incumbent) takes office.

We begin with a canonical model of legislative bargaining due to Baron and Ferejohn (1989) and its generalization to other policy environments. We then consider a related model that explores the impact of institutional features on the cohesion of legislative coalitions. We next turn attention to an alternative model of coalition formation that illustrates government formation in a parliamentary system. In each of these models, a minimum winning coalition is typically formed in equilibrium; we subsequently examine two alternative explanations for the supermajorities often observed in practice.