Why do some interest groups lobby politicians and others lobby bureaucrats? We theorize lobbying venue choices and intensity as a function of contract enforceability with policy makers, politicians, or bureaucrats. We argue that organizational structures of interest groups, in particular, whether they are centralized or decentralized, substantially affect their lobbying strategies because they are associated with different ability to monitor and enforce contracts with policy makers and punish them when they fail. We further demonstrate that the effect of centralized versus decentralized structure on venue choices is conditional on the types of electoral system: majoritarian, semiproportional (single, nontransferable vote: SNTV), or proportional representation systems. We test this argument using longitudinal survey data on lobbying which span two decades and cover around 250 interest groups in various sectors and issue areas in Japan. The results lend strong support to our argument about contract enforceability under alternative electoral systems.