Analysis of three cross-sectional polls administered by the Gallup Organization at 10-year intervals—in 1990, 2000, and 2010—demonstrates that partisan identification has become an increasingly important determinant of environmental concern within the American mass public. Polarization on global warming is especially clear, even when compared to a variety of other social, economic, and political problems, but party sorting seems to occur only as citizens acquire information and become familiar with elite cues. The implications of this for the U.S. environmental movement and the strategies it employs are discussed.