We study, theoretically and empirically, legislative influence over executive agencies, focusing on the breadth of agency responsibilities. We model interest groups, the legislature, and agencies. Politicians exert costly effort to influence agencies in exchange for interest groups' campaign contributions. Effort, however, can only be imperfectly targeted. When effort is spent on behalf of one group, some spills over to benefit other interest groups. This creates externalities of influence that are larger in broad agencies, deterring legislative influence. Empirically, we develop a novel lobbying-based measure of breadth and combine it with survey data on influence in 70 US federal agencies. Broad agencies report less influence, and we rule out several alternative explanations. These results are important for understanding how to insulate divisive tasks from political influence.