Anyone who has watched the bitter competition between the Democrats and Republicans in Congress in recent years, or the fight to win the Republican nomination for President this year, might be wondering how to explain the current political party system in the United States. It looks like a period of deep ideological cleavages between the parties, and pretty strict enforcement of some form of ideological correctness at least in one of them. Yet for decades or even centuries scholars of politics have argued that ideological divisions were relatively weak in our elections and legislative process. What is happening, and what has happened? This may be the central question for understanding the current state of American politics. So it will be a special pleasure to welcome to campus, on April 17, one of the leading and most original scholars of both parties and public opinion in the country, John Zaller. Zaller’s book on _The Nature and Origins of Mass Opinion_ (1992) is widely ranked with just a few other works, such as The American Voter, as a classic in the study of citizen attitudes. In _The Party Decides: Presidential Nominations Before and After Reform_ (2008), John and colleagues have made an argument about how party elites, including interest groups aligned closely with one or the other party, shape presidential nominations. Their argument has become one of the tools used by commentators seeking to handicap the current nomination process. Now he is delving more deeply into the nature of political parties in the United States, both now and in the past. As in his other work, a central theme is the relationship between mass opinion and political leadership. And that has to involve where ideology fits in. Are we in an era where ideology is much more central to parties? Why, and with what consequences? Scholars and the public have had many different views of this question. In one traditional view, they were gangs of entrepreneurs seeking to capture and divide the spoils of office. Broad principles were to be adapted and adopted only as useful. For many years political scientists who believed this view also criticized it. They argued that, instead, parties should be organized around clear principles so that voters would be given clear choices about the ideology that would govern them – an idea called “responsible party government.” A third view argued, in ways that might look more plausible now than in the past, that reducing American politics to a fight about principles between just two sides couldn’t possibly represent a huge country fairly, would leave many voters feeling unrepresented, and maximize conflict. From this view, truly partisan divisions were dangerous, and a politics based on interest groups would be both more representative and less dangerous. The latter view assumes, however, that interest groups create cross-cutting cleavages: that peoples’ identities mean they agree with lots of other people on different things. What happens, however, if the interest groups become more ideological, and more closely linked to parties? What happens if voters are more easily mobilized on broader, ideological issues than narrower (“save the whales”) issues? More precisely, what happens when one party sees “save the whales” as an attack on its basic principles? It is getting harder to argue that political parties are not so important, but not much easier to figure out what they do or how they do it. The central questions have to do with the roles of voters and public opinion, organized interests, and politicians. What motivates each, and what influence does each have? Nobody has contributed more than John Zaller has to our understanding of these questions.