Firms often oppose costly public policy reforms—but under what conditions may they come to support such reforms? Previous scholarship has taken a predominantly static approach to the analysis of business positions. Here, we advance a dynamic theory of change in business policy positions that explains how business may shift from opposing to supporting new regulation over the course of multiple rounds of policymaking. We identify three sets of drivers and causal mechanisms behind business repositioning related to political, policy, and market change. We argue that political mechanisms can shift opposition to “strategic support” for reform, whereas policy and market mechanisms may shift opposition or strategic support toward “sincere support.” We examine the reconfiguration of business interests and policy positions in the context of three decades of US climate politics, focusing on the oil and gas, electricity, and auto sectors. Our dynamic theory of business positions moves beyond the dualism that views business as either opposing or supporting public interest regulation. We thus advance our understanding of why initial business opposition can incrementally turn into strategic or sincere support for policy reform.