Anti-reclamation movements are common in Indonesia, but their effectiveness varies. Such movements, which oppose the infilling of coastal waters and wetlands, consistently draw support from environmentalists, fisherfolk, and coastal residents. To succeed, however, they must transcend these constituencies and mobilize broad coalitions. In this paper, I apply the concept of political opportunity to explain variation in the ability of anti-reclamation movements to achieve this goal. Specifically, I argue that the opportunity to build broad coalitions depends on the positioning of political, economic, and communal elites. Disagreement among these groups creates opportunities for activists to recruit some of them as allies in the construction of economically diverse, cross-class coalitions. Consensus, by contrast, excludes elites as potential allies, forcing activists to build geographically expansive but class-based coalitions. To develop my argument, I draw on local news archives and primary source documents to compare similarly situated anti-reclamation movements in Bali and Makassar. In Bali, the movement flourished by cultivating an alliance with communal elites and local businesses. In Makassar, the movement withered because public officials, local businesses, and communal elites all welcomed reclamation. My findings imply that anti-reclamation movements are most likely to succeed when they emphasize communal identities with cross-class appeal. Yet such tactics alienate parallel movements from one another and undermine national activism. As a result, anti-reclamation movements fight the same battles over and over without achieving national reforms that would empower coastal communities to participate in coastal planning. Under such conditions, reclamation deepens the vulnerability of coastal communities to climate change.