We propose a theory of political parties in which interest groups and activists are the key actors, and coalitions of groups develop common agendas and screen candidates for party nominations based on loyalty to their agendas. This theoretical stance contrasts with currently dominant theories, which view parties as controlled by election-minded politicians. The difference is normatively important because parties dominated by interest groups and activists are less responsive to voter preferences, even to the point of taking advantage of lapses in voter attention to politics. Our view is consistent with evidence from the formation of national parties in the 1790s, party position change on civil rights and abortion, patterns of polarization in Congress, policy design and nominations for state legislatures, Congress, and the presidency.

What is a party? This article presents the argument that rmal party apparatus is only one part of an extended network of interest groups, media, other advocacy organizations and candidates. The authors have measured a portion of this network in the United States systematically by tracking lists of names transferred between political organizations. Two distinct and polarized networks are revealed, which correspond to a more liberal Democratic group and a more conservative Republican group. Formal party organizations, like the Democratic National Committee and the Republican National Committee, tend to receive information within their respective networks, which suggests that other groups serve to funnel information towards the formal party.

Political Ideologies and Political Parties in America puts ideology front and center in the discussion of party coalition change. Treating ideology as neither a nuisance nor a given, the analysis describes the development of the modern liberal and conservative ideologies that form the basis of our modern political parties. Hans Noel shows that liberalism and conservatism emerged as important forces independent of existing political parties. These ideologies then reshaped parties in their own image. Modern polarization can thus be explained as the natural outcome of living in a period, perhaps the fi rst in our history, in which two dominant ideologies have captured the two dominant political parties.

An increasing number of scholars are using longitudinal social network data to try to obtain estimates of peer or social influence effects. These data may provide additional statistical leverage, but they can introduce new inferential problems. In particular, while the confounding effects of homophily in friendship formation are widely appreciated, homophily in friendship retention may also confound causal estimates of social influence in longitudinal network data. We provide evidence for this claim in a Monte Carlo analysis of the statistical model used by Christakis, Fowler, and their colleagues in numerous articles estimating "contagion" effects in social networks. Our results indicate that homophily in friendship retention induces significant upward bias and decreased coverage levels in the Christakis and Fowler model if there is non-negligible friendship attrition over time.We thank Nicholas Christakis and James Fowler for generously sharing the simulation code adapted in this paper and for helpful comments and clarifications. We are also grateful two anonymous reviewers for helpful suggestions and feedback.

What are the primary factions within the Democratic and Republican parties, and to what extent do rival factions cooperate? We address these questions using a unique data set of information sharing between party organizations, media outlets, 527s, and interest groups. Using social network methods, we identify two major information-sharing clusters, or expanded party networks; these networks correspond to a liberal/Democratic grouping and a conservative/Republican grouping. We further identify factions within each party network, but we find a high degree of cooperation between party factions. That is, our data suggest that beneath the intraparty disagreements we observe in primary elections and policy debates there is a subterranean pattern of organizational cooperation.