Pre-electoral Coalitions and Post-election Bargaining1

We study a game-theoretic model where political parties can form coalitions both before and after the elections. Before election, coalitions can commit to a seat-sharing arrangement, but not to a policy or to a division of rents from o¢ ce; coalition members are free to break up and join other coalitions after the election. Equilibrium pre-electoral coalitions are not necessarily made up of the most ideologically similar parties, and they form under proportional representation as well as plurality rule. They do so to avoid "splitting the vote", but also because seat-sharing arrangements will in‡uence the ex post bargaining and coalition formation.