Concern that lawmakers grant preferential treatment to individuals because they have contributed to political campaigns has long occupied jurists, scholars, and the public. However, the effects of campaign contributions on legislators’ behavior have proven notoriously difficult to assess. We report the first randomized field experiment on the topic. In the experiment, a political organization attempted to schedule meetings between 191 Members of Congress and their constituents who had contributed to political campaigns. However, the organization randomly assigned whether it informed legislators’ offices that individuals who would attend the meetings were contributors. Congressional offices made considerably more senior officials available for meetings when offices were informed the attendees were donors, with senior officials attending such meetings more than three times as often (p < 0.01). Influential policymakers thus appear to make themselves much more accessible to individuals because they have contributed to campaigns, even in the absence of quid pro quo arrangements. These findings have significant implications for ongoing legal and legislative debates. The hypothesis that individuals can command greater attention from influential policymakers by contributing to campaigns has been among the most contested explanations for how financial resources translate into political power. The simple but revealing experiment presented here elevates this hypothesis from extensively contested to scientifically supported.