To understand polarization we must understand political parties. Polarization is often considered abnormal and even pathological. Yet this assumption is questionable. Polarization is typically defined as growing cohesion in the policy stands of parties' officials, activists, and voters and divergence in parties' policies (Fiorina and Abrams 2008). A comparative and historical perspective reveals that cohesive parties with divergent policy positions are common in stable democracies. Parties draw support from different societal interests, taking on divergent policy positions as a result (Karol 2009). Party competition also generates “teamsmanship” (Lee 2009), as politicians seek to discredit the other side while claiming credit for themselves. As a result, clear divisions between parties are the norm.

In countries other than the United States, however, strong parties do not produce the dysfunction now visible in Washington. The problem is the mismatch between polarized parties and the U.S. Constitution's separation of powers. This tension was obscured for decades when parties were atypically divided and, earlier still, when the American state did little. The parties' increasing cohesion was unplanned, as was their mid-twentieth-century eclipse. Polarization may abate again, but there is little reason to believe such a development is imminent or that tinkering with redistricting, primaries, and campaign finance law will hasten it. Recognition of polarization's durability could eventually produce openness to reforms that are now beyond reach, such as a move toward a parliamentary system. In the medium term, however, abolishing the filibuster is a more realistic goal.

AMERICAN PARTIES: PAST AND PRESENT

Traditionally, political scientists have seen American parties as pragmatic and election oriented. For some leading party scholars, parties and interest groups were competing forms of political organization. E. E. Schattschneider saw the American political system plagued by “pressure groups” that he thought disproportionally represented the wealthy.