Historically, the emergence of mass politics has depended upon a minimum of social heterogeneity (LaPalombara and Weiner 1966; Dahl 1966; Lipset and Rokkan 1967). Religious, economic, ethnic, linguistic, and regional differences provided social cleavages along which organizations, especially political ones, developed. The number, salience and centrality, and political significance of the cleavages varied among societies but the existence of differences, their expression as groups within the larger society, and their politicization are virtual constants. In all mass democracies, not excluding the United States, parties are instruments of collective action through which groups promote and protect interests which are not satisfied by the usual operation of the social structure and markets. As a result, groups provide the working politician with a guide to the elec- torate, and it is the rare one who deals with it in any other fashion. The variety of groups with which parties and office-seekers must deal varies greatly among societies. In some cases the lines of cleavage are few and, relatively speaking, simple: parties coincide with a few groups, and sometimes only one. In other cases, supporters of a party are religiously, ethnically, racially, linguis- tically, and class heterogeneous; no group, however defined, represents more than a fraction of a party's supporters (Rose and Urwin 1969). Whether their base is heterogeneous or homogeneous, politicians reinforce it through their appeals to the electorate. The heterogeneity of a party's support is, however, an important variable. Parties with a homogeneous clientele present a homogeneous programmatic face to the electorate because, ceteris paribus, the interests and concerns of their supporters are more focused. Heterogeneous par- ties, by contrast, enjoy agreement on a smaller number of issues because, again, ceteris paribus, the social and economic differences which divide their supporters also promote inconsistent issue agendas among them. The apparent programmatic vagueness of the American parties reflects the diversity of their coalitions. Southern California is an appropriate observatory for the study of party coalitions because it offers an opportunity to examine further some political con- sequences of social heterogeneity. Two issues are examined in this paper. First, how is the social diversity of the electorate represented in the parties? Do the parties mobilize groups different from those in the national parties? Do the "new" ethnics have a particular impact on the coalitions? Second, how do the local parties represent the policy agendas of the groups that constitute their support base?
