We consider a spatial model of electoral competition between two political parties. These parties are incompletely informed about voter preferences and search adaptively, by means of polling, for those policy platforms which might make them win the election. We introduce interest groups in this spatial framework. Di§erent interest groups coordinate voting behavior and transmit information about voter preferences to the political candidates. Moreover, organization of voters into di§erent interest groups occurs endogenously. We investigate the dynamics of this spatial model of electoral competition by looking at the mean-dynamics, i.e. by replacing stochastic variables by their expected values. The resulting Markov process shows that voting cycles exist. The mechanism driving these voting cycles may explain some empirical regularities found in the political science literature.