Partisan swing measures how much a party improved or declined in vote share from one comparable election to the next. In special election analysis, swing is the primary metric used to aggregate results across diverse districts—it controls for structural partisan lean (measured by Cook PVI) and isolates pure voter sentiment change. A consistent partisan swing of the same direction across 30 or more special elections signals a change in national political environment. Political scientists distinguish swing from raw vote share because swing reveals momentum regardless of whether the swinging party actually won.