This paper investigates whether firms donate to political campaigns in order to influence supply in one of the largest markets in the U.S.—housing. A model of electoral competition and special interest politics highlights two mechanisms for this to occur: firms can be buying favors or supporting policy. To assess which mechanism dominates, I investigate residential construction firms that donate to mayoral candidates and their impact on local housing supply. To do this, I collect the first large-scale dataset of campaign donors in U.S. mayor's races. I employ a firm-level regression discontinuity (RD) design in close mayoral races to evaluate the mechanism of buying favors. Donating to a narrowly elected mayor substantially increases that firm's growth of new property sales by $243.8 thousand per year. Favors increase for bigger donations, are concentrated among powerful mayors, and matter more for incumbent firms. Participating in local politics does entail political risk; donors to the runner-up experience declining sales compared to firms that do not donate in mayor's races. Otherwise, mayors do not appear to bar entry of their donor's competitors. I then evaluate the mechanism of supporting policy by exploiting an aggregate, city-level RD design. Pro-development mayors that attract more construction donors more than double new housing permits citywide over five years. This total effect dwarfs the sum of private favors to donors, as even non-supporters sell more. Taking these estimates together, 70.1% of the impact of a pro-development mayor on housing supply is due to differences in housing policy between candidates. Translating the effect into levels, 10.2% of total permits issued every year in an American city are favors to political donors.