Modern clientelist exchange is typically carried out by intermediaries—party activists, employers, local strongmen, traditional leaders, and the like. Politicians use such brokers to mobilize voters, yet little about their relative effectiveness is known. The authors argue that broker effectiveness depends on their leverage over clients and their ability to monitor voters. They apply their theoretical framework to compare two of the most common brokers worldwide, party activists and employers, arguing the latter enjoy numerous advantages along both dimensions. Using survey-based framing experiments in Venezuela and Russia, the authors find voters respond more strongly to turnout appeals from employers than from party activists. To demonstrate mechanisms, the article shows that vulnerability to job loss and embeddedness in workplace social networks make voters more responsive to clientelist mobilization by their bosses. The results shed light on the conditions most conducive to effective clientelism and highlight broker type as important for understanding why clientelism is prevalent in some countries but not others.