Lobbyists are omnipresent in the policymaking process, but the value that they bring to both clients and politicians remains poorly understood. We develop a model in which a lobbyist's value derives from his ability to selectively screen which clients he brings to a politician, thereby earning the politician's trust and preferential treatment for his clients. Lobbyists face a dilemma, as their ability to screen also increases their value to special interests, and the prices they can charge. A lobbyist's profit motive undermines his ability to solve this dilemma, but an interest in policy outcomes—due either to a political ideology or a personal connection—enhances it, which paradoxically increases his profits. Using a unique dataset from reports mandated by the Foreign Agents Registration Act, we find that lobbyists become more selective when they are more ideologically aligned with politicians, consistent with our prediction.