We study the effects of ideological polarization on regional planning networks. Over the last several decades, Americans have sorted themselves into local communities that are increasingly homogenous in their partisan and ideological make-up (Bishop 2008). Local governments from these communities face immense pressures to engage in regional planning; however, we hypothesize that differences in the political composition of local constituencies will render such intergovernmental cooperation difficult. Using data from a recent survey of California planners and government officials, we map regional planning networks in five California regions in real geographic space and test hypotheses about the factors that lead local governments to engage in regional planning activities. We find that, after controlling for physical distance and similarity of planning preferences, local governments whose constituents are similar politically are more likely to cooperate with one another in regional planning efforts than those whose constituents hold disparate political views.