A model of endogenous party membership and platforms with opportunistic politicians

We model endogenous party membership and party platforms when party activists have different policy preferences and different priorities. In our model, party activists are heterogeneous not only in their ideal policy but also in their intensity to pursue policy issues vis-a-vis the spoils from office. We provide existence and characterization results for an equilibrium in which (ⅰ) party activists choose their party affiliation with correct anticipation of the choices by others, and (ⅱ) the resulting party platforms are consistent with the affiliation choice. This model offers an explanation of the overlaps in ideology between political parties. It also provides insights into the inner composition of political parties with different size.