Several mechanisms may contribute. Sorting moves liberals increasingly into the Democratic coalition and conservatives into the Republican coalition, reducing the number of liberal Republicans and conservative Democrats. Geographic concentration changes the incentives of districts and primaries. Activists, donors, media environments, and strategic choices by leaders can reward sharper conflict. No one mechanism should be treated as a complete explanation in every period.

Polarization has mixed institutional consequences. Clearer party differences can make labels more informative and help voters assign responsibility. The same clarity can make compromise costly, intensify disputes over rules, and encourage each side to view concessions as betrayal. More party-line voting is evidence consistent with polarization, but one contested vote cannot establish a long-term trend; the concept requires comparison across issues, time, and multiple institutions.

Polarization is not divided government, dealignment, or affective polarization. Divided government describes who controls institutions. Dealignment describes weaker attachment. Affective polarization concerns social dislike between partisan groups, which can increase even when policy positions do not move equally far apart. Researchers therefore compare multiple votes, policy scales, and time periods rather than inferring polarization from angry campaign language alone. Separated party positions or legislative voting records indicate ideological polarization; social dislike indicates affective polarization; and a more consistent match between ideology and party indicates sorting.

Video lesson: Party Systems: Crash Course Government and Politics #41