Much of the lobbying process is inherently competitive: when lobbyists with opposing goals attempt to move outcomes in their preferred direction, successful lobbying by one actor will disadvantage opposing actors. This article theorizes and quantifies the indirect form of influence that competing lobbying actors exert on each other. While existing theories of competitive lobbying have focused on legislation, we argue that all stages of the lobbying process involve competition. Our findings make two contributions to the study of lobbying influence. First, using spatial econometrics, we present the first estimates of how the success of one lobbying actor is shaped by the lobbying activities of opposing actors. Second, we study competition in three diverse empirical settings that capture three different stages of the lobbying process: (1) lobbying camps favoring opposite legislative outcomes in five European countries, (2) US lobbying firms competing over client resources, and (3) corporations competing for administrative trade barriers in 19 World Trade Organization member countries. The results reveal important insights about how interdependence among lobbyists conditions their effectiveness. Our application of spatial techniques to model interdependence between actors is useful for all scholars who want to take competitive or collaborative diffusion mechanisms into account in studies of lobbying and public policy.