Political brokers mobilize voters all over the world, yet little is known about what motivates them to do so. This article theorizes about two drivers of brokers' efforts: (1) incentives—monetary rewards or sanctions—and monitoring and (2) partisan attachment. We examine our theory using data on the Mexican National Educational Workers Union (SNTE), Latin America's largest union and a well-known political machine. Consistent with the role of teachers as brokers, we find that the vote share of parties supported by the SNTE machine is higher in polling stations located in schools. This effect is absent when teachers are asked to mobilize voters in support of a party for which they have no partisan attachment, and it is uncorrelated with the union's monitoring capacity. This suggests that partisan attachment, rather than incentives and monitoring, explains the SNTE's effectiveness as a political machine.