Climate change activism has been uncommon in the U.S., but a growing national movement is pressing for a political response. To assess the cognitive and affective precursors of climate activism, we hypothesize and test a two-stage information-processing model based on social cognitive theory. In stage 1, expectations about climate change outcomes and perceived collective efficacy to mitigate the threat are hypothesized to influence affective issue involvement and support for societal mitigation action. In stage 2, beliefs about the effectiveness of political activism, perceived barriers to activist behaviors and opinion leadership are hypothesized to influence intended and actual activism. To test these hypotheses, we fit a structural equation model using nationally representative data. The model explains 52 percent of the variance in a latent variable representing three forms of climate change activism: contacting elected representatives; supporting organizations working on the issue; and attending climate change rallies or meetings. The results suggest that efforts to increase citizen activism should promote specific beliefs about climate change, build perceptions that political activism can be effective, and encourage interpersonal communication on the issue.