Efforts to study the dynamics of legislative and party politics in developing countries often confront a serious obstacle – lack of public data on legislative actions. This paper proposes a new feasible approach for obtaining legislative information indirectly – analyzing the perceptions and lobbying behaviors of business interest groups who are experienced, knowledgeable and highly invested in staying informed on policymaking processes. I build on extant literatures on political parties and lobbying to show how we can systematically exploit group insights to study legislative parties in data-scarce environments and complement existing approaches in data-rich cases. I then demonstrate and evaluate this approach by using original data from a 2006 survey of 158 groups in a data-rich case, Brazil, to mediate existing scholarly debates regarding the sources of legislative unity of Brazilian parties.