We model endogenous party membership choice and party platform formation when party activists differ not only in their policy preference but also in the intensity with which they pursue policy issues vis-a-vis the spoils from electoral victories. We provide existence results for equilibria 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 for the overlaps in ideology between political parties. It also provides insights into the internal composition of political parties with different sizes.