This article develops a framework to explain ideological diversity within political parties in parliamentary democracies from the positions of individual legislators. First, I review the different theories explaining the variation of ideological diversity within political parties in the field of party politics and legislative studies. Then, I propose to model the relations between legislators, their party and their constituents as a competitive delegation process building on the principal-agent theory. I draw on the literature on the distribution of power within political parties to argue that intra-party ideological diversity can best be explained by vertical bargains taking place between the different territorial layers of political parties.