Can interest groups influence elections? Evidence from unions in Great Britain, 1900–2019

Abstract Unions sponsor electoral candidates around the world, yet little is known about the consequences of these arrangements. I study how union sponsorship affected the electoral prospect of British parliamentary candidates throughout the 20th century. I collect new data on the universe of union‐sponsored candidates. Employing a difference‐in‐differences design based on within‐candidate variation induced by the sponsorship institution and its abolishment, I document that sponsorship caused a six percentage point increase in candidate vote shares. I outline theoretical mechanisms and examine whether sponsees improved their electoral fortune because of changes in constituencies, opponents, resources, mobilization, or information. The evidence supports the constituency and resource mechanisms: Sponsorship helped candidates get nominated in attractive constituencies, accounting for two‐thirds of the effect, and caused an inflow of resources into constituency–party organizations. Overall, sponsorship promoted the representation of union‐friendly candidates in parliament, but it only led to moderate shifts in the balance of power between parties.