Scholarship commonly implies that the major political parties in the United States are configured as mirror images to each other, but the two sides actually exhibit important and underappreciated differences. The Republican Party is primarily the agent of an ideological movement whose supporters prize doctrinal purity, while the Democratic Party is better understood as a coalition of social groups seeking concrete government action. This asymmetry is reinforced by American public opinion, which favors left-of-center positions on most specific policy issues yet simultaneously shares the general conservative preference for smaller and less active government. Each party therefore faces a distinctive governing challenge in balancing the unique demands of its base with the need to maintain broad popular support. This foundational difference between the parties also explains why the rise of the Tea Party movement among Republicans in recent years has not been accompanied by an equivalent ideological insurgency among Democrats.