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.