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 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 U.S. elections and legislative process. What is happening, and what has happened? Professor John Zaller of UCLA is one of the leading scholars of both political parties and public opinion in the country and here he explores these questions central to understanding the current state of American politics.