By-elections in constituencies such as Orpington, Bermondsey, and Govan generate headline news in England and raise fundamental questions concerning why third parties manage unexpected breakthroughs, and why the government consistently loses support through the "mid-term blues." The first systematic study of British by-elections, this book addresses these questions through an analysis of postwar trends in party support. The first section addresses changes in campaigns, contrasting the stable by-elections of the postwar decade with the more volatile races characteristic of today. The second section explores systematic trends in the light of theories of partisan alignment and retrospective voting and analyzes the influence of the role of the candidate, party organization, the media, and opinion polls. With an essential reference section listing changes in party support in almost four hundred British by-elections since 1945, this will be the standard work of reference on a subject of continuing significance on the British political scene.