Usually the provision of international environmental public goods cannot be secured by a single state. Rather, a group of major powers has to pool its resources to provide structural leadership in order to achieve an effective regime. Such a group of pushers uses its structural power to achieve its goal. However, it faces two challenges. First, it may have to overcome the opposition of a group of laggards that desires less environmental protection and may try to counter the pushers' efforts. We hypothesise that the regime will be more effective to the extent to which the pushers predominate over the laggards in terms of structural power. Second, both groups may have to overcome a collective action problem with regard to dispensing costly side-payments. We argue that social capital embedded in inter-state networks may help the groups to overcome such collective action problems. Thus we argue that the regime will be more effective to the extent to which pushers are predominant and also have more social capital than laggards. Empirical results support our hypotheses.