Tackling global challenges such as climate change requires not only the capacity to act but also the belief that action will be effective. Across two large, nationally representative U.S. samples (Study 1: N = 1,197; Study 2: N = 31,324), we show that response efficacy—the belief that actions will work—plays a central role in mobilizing climate engagement. Study 1 demonstrates that governmental response efficacy is the strongest predictor of climate action across behavioral domains, including public advocacy, political participation, financial contributions, and lifestyle change. While Republicans reported lower efficacy overall, their engagement was especially responsive to efficacy beliefs, highlighting a cross-partisan pathway to mobilization. Study 2 evaluates 17 candidate communication strategies and identifies a single, scalable mechanism: emphasizing the collective efficacy and emotional benefits of climate action. This approach consistently increased personal, collective, and governmental efficacy across ideological groups. Together, these findings reveal governmental response efficacy as a key psychological driver of climate action and provide a practical intervention for strengthening it. By clarifying the structure of efficacy beliefs and demonstrating how to enhance them, this research offers new insight into how societies can foster broad-based public engagement in addressing urgent environmental crises.