This article investigates the nature of policy path dependence through analysis of climate policy formation in the United States. In 2008 the US Congress attempted to pass the Lieberman–Warner bill, a comprehensive climate and energy package that would have capped greenhouse emissions and established a nationwide cap and trade program. In the same year, California successfully enacted the Global Warming Solutions Act. This article explores the circumstances of both cases and raises the question of why legislation at the state level was successful and took such a divergent form from legislation at the federal level. The divergence of these cases is used to highlight the nature of coalition formation and policy path dependence in the legislative process. Explanations of policy tend to gravitate toward either the generalizability of game theoretic approaches or the empirical depth of case studies. This article suggests a combined approach that uses case studies to analyze the positions and motivations of actors and to then model policy development over time. The approach examines policy through the formation and negotiation of policy coalitions. Drawing on the Advocacy Coalition Framework and omnibus analysis, the approach expands these coalition theories first by analysing legislative development at the interface of legislators and constituent interest groups, and second by adding temporal dimension to the analysis. The findings suggest that policy is path dependent in that it is negotiated between coalitions that in turn create stability in the policy process and insulate policy fields from external shocks. Policy path dependence suggests that theory alone is insufficient to predict policy outcomes; policy results depend strongly on prior policy efforts, historically and socially contingent coalitions, and the resulting framing of policy possibilities.