Most American congregations belong to denominations. These national networks often promote relevant political issues that constituent congregations may choose to address. Therefore, religious organizations might play a significant role in promoting the process of policy diffusion in the U.S. states. At the same time, congregations are tradition-maintaining institutions that focus first on their own immediate concerns, promoting the maintenance of distinctive regional political contexts and reinforcing the variation of federalism. In this article, we assess the role that religious organizations play in American federal democracy, promoting diffusion or the status quo, with respect to one crucial issue on which religious groups have been particularly vocal: the environment and global warming. We employ two datasets (a large-N national survey and two in-depth survey case studies of clergy in Ohio and South Carolina) and find evidence that religious organizations are vast communication networks that can promote policy diffusion, but typically do not.