Third-Party Strategy under Plurality Rule: The British Liberal Democrats and the New Zealand Social Credit Party

This article examines the strategic options facing small centrist third parties in two-party parliamentary systems operating under the single-member district plurality electoral system. It uses a spatial model to show that centrist third parties are better off targeting the ‘safe’ districts of a major party rather than marginal districts. Furthermore, it is optimal to target one party’s districts, not both, to benefit from tactical and protest voting. This article also questions the implicit conclusion of the median-legislator theorem that pivotality-seeking is the best strategy for a third party, at least under the single-member district plurality system, because that would usurp voters’ ability to select the executive directly, a key feature of two-partism. Finally, this article shows that third parties can damage themselves if they ‘flip’ their strategies from opposing particular major parties to supporting them. Evidence is provided for the British Liberal Democrats and New Zealand’s historic Social Credit Party.