SAFE SEATS,MARGINAL SEATS,AND PARTY PLATFORMS: THE LOGIC OF PLATFORM DIFFERENTIATION*

This paper analyzes a spatial model of two-party competition where parties are not monolithic decision makers but collections of self-motivated officeholders. Party platforms are chosen collectively by incumbent officeholders. The main result is that in a stable equilibrium party platforms do not converge to the same point. Instead, the parties choose platforms so that voters can distinguish between them, and these platforms divide the set of legislative districts cleanly along party lines. All incumbents prefer this situation to one where the platforms converge, because it improves their own chances of reelection.