Here we document an under-studied but important phenomenon that we call ascendant public opinion, which emerges when a new concern is framed as an instance of a broader issue and gains ascendancy over that issue in the public’s mind. We focus on the ever-increasing role climate change has come to play over the past three decades in shaping how Americans think about broader environmental concerns. We show that news coverage of the environment has focused increasingly on climate change over time, while climate change concurrently has come to dominate all other environmental problems in the strength of its association with general environmental concern in opinion surveys. Panel studies provide evidence that the growing correlation between attitudes on climate change and the environment is predominantly due to the impact of the former on the latter. These developments have been consequential: we estimate that Americans’ level of concern about the environment is now both more elevated and more polarized along party lines than if climate change did not occupy its dominant place on the environmental agenda. Climate change is likely just one example of how ascendant public opinion can have important consequences for politics and policy.