This paper studies a uni-dimensional model of electoral competition between two parties with two types of politicians. 'Opportunist' ones care only about the spoils of the office, and 'militant' ones have ideological preferences on the policy space. Opportunist politicians review their affiliations and may switch parties, seeking better election prospects. In this framework, we compare a winner-take-all system, where all the spoils of office go to the winner, to a proportional system, where the spoils of office are split among the two parties in proportion to their vote shares. We study the existence of short term political equilibria and the dynamics and stability of policies and of party membership decisions. In the long run, it is possible that proportional systems see opportunist politicians spread over all parties, but this situation is unstable in winner-take-all systems.