Why are some legislators more effective than others in fragmented presidential systems? I argue that in Brazil’s fractionalized party system, legislative member organizations (LMOs) supply policy and political information that parties often lack, enabling lawmakers to advance bills. I test this claim using novel legislative effectiveness scores (LESs) for sponsors and rapporteurs in Brazil’s lower chamber. Quantitative results show that LMO affiliation is associated with higher effectiveness, but only in highly structured organizations. Public security LMOs boost both sponsorship and rapporteurship, while agribusiness LMOs increase rapporteurship effectiveness. Weakly organized LMOs show null effects. Party affiliation matters, but parties do not consistently provide information and coordination. Qualitative data identify two mechanisms by which strong LMOs operate: placing aligned members in key positions and leveraging expertise to shape agendas and voting cues. These findings recast effectiveness in Brazil as a function of cross-party informational networks rather than parties alone and identify scope conditions under which LMOs matter in other multiparty presidential democracies.