An unpredictable pendulum: UK electoral dynamics in the twenty-first century

Abstract Post-war electoral dynamics were framed by a two-party system that usually provided single party governments with a legislative majority. These conditions promoted a pendulum-like swing of votes between two parties that alternated in office. The electorate responded ‘thermostatically’ to governments by moving in the opposite direction to policy and imposed the ‘costs of ruling.’ The major parties competed by moderating their positions and building reputations for competence. They vacated the centre in the 1970s and suffered a loss of their reputations for competence, leading to an increase in the centre-party vote. When the Liberal Democrat vote collapsed after 2010, niche parties, such as Reform UK, the Greens, and the Scottish National Party, picked up support. These developments have produced increasingly complicated dynamics. The major parties now risk losing votes to parties on their flanks. This reduces the appeal of moderation and causes uncertainty about strategy. Governments still suffer from thermostatic effects and the costs of ruling, creating opportunities for a wider range of opposition parties. These dynamics will produce volatility that the plurality electoral system will amplify to produce even more astonishing electoral turnarounds, and complicate strategic decision-making for parties.