In the absence of federal legislative action, states and even cities are moving to fight climate change—individually and in regional alliances—while other states sit out. When California announced last year that it will end sales of gas-powered vehicles by 2035, a dozen other states followed suit. Now, seeking to undo four years of environmental deregulation, the Biden administration is pressing for climate action. What happens to the state-federal dynamic when the federal executive branch shifts its position on climate change?