Why does issue linkage gain support from some domestic groups, not others? Al- though governments have long used economic tools to promote environmental and climate goals internationally, we know relatively little about when such linkage gains support or elicits backlash from environmental activists. Based on the premise that ac- tivists face severe commitment problems during linkage processes, I argue that activists’ positions on linkage vary by their ties to intergovernmental organizations (IGOs) and non-governmental organizations (NGOs). Focusing on the US’s trade-environmental linkage, I theorize that IGO ties tend to mitigate the commitment problem and facili- tate cooperation by providing compliance information useful for enforcement, whereas NGO ties incentivize activists to increase the stake of their demands due to loyalty considerations. Quantitative analyses of new data on environmental NGO positions on US trade agreements, along with archival evidence and case studies of two NGOs, support the theory. This paper shows that domestic groups’ external ties to other non-state actors play different roles in shaping of successful cooperation in a complex international system.