• renewable portfolio standard;
  • electricity market;
  • energy policy;
  • policy adoption;
  • citizen ideology;
  • renewable energy

Renewable energy policy has far-reaching implications for national and international economic, environmental, and political sustainability, but thus far within the United States it has been almost entirely the province of state governments. This article examines the factors motivating state-level policymakers to adopt different forms of a renewable portfolio standard (RPS), highlighting the distinction between degrees of policy stringency, ranging from entirely voluntary participation to rigorous and strictly enforced targets. In the process we introduce a new metric for assessing stringency, more precise and reliable than the various proxies used previously, and analyze its relationship to drivers of policy adoption. We find that policies of different stringencies are motivated by systematically different underlying factors. State-level citizen political ideology is a significant predictor of RPS policy adoption, particularly for “voluntary” and “weak” policy designs. “Strong” policy designs, on the other hand, are best predicted by ideology at the government level, i.e., the degree of institutional liberalism. These findings may inform current implementation and program evaluation efforts, and potentially point the way toward more effective policy choices if and when an RPS moves forward on the national policy agenda, while the stringency metric central to this analysis can be of use to other policy scholars concerned with topics both within and beyond the realm of energy policy.