Widespread adoption of the Internet has transformed how most US political advocacy organizations operate, but perhaps more important has been the formation of new types of advocacy organizations. These ‘Internet-mediated advocacy organizations’ tend to have smaller, geographically dispersed and networked staffs, behave as hybrids of traditional political organizations, and emphasize the use of online tools for offline action. The climate change debate has spurred formation of many such organizations – including 350.org – that now advocate for climate action alongside legacy/environmental organizations. How do these organizations differ from their legacy/environmental counterparts? What does their rise mean for climate change political advocacy? I explore these and other questions through in-depth interviews with top online strategists and other staffers at Environmental Defense Fund, Natural Resources Defense Council, Sierra Club, Greenpeace USA, Energy Action Coalition, 1Sky, and 350.org. Interviews revealed broad agreement among Internet-mediated/climate groups regarding core strategic assumptions about climate advocacy, but some divergence among legacy/environmental organizations. They also revealed connections between these assumptions, audience segment targeting, and strategic use of the Internet for advocacy. I discuss implications for the future of US climate advocacy.