In the last two decades, many U.S. states introduced policies to promote electricity generation from renewable energy sources (RES‐E). Renewable portfolio standards (RPS) are considered to be the key RES‐E policy tool to date. This article tackles the question on why some state legislators were front‐running the trend of RPS implementation whereas others adopted policies just recently, and why others have not adopted them at all. We compile data on financial contributions of conventional energy interest groups (CEI) and renewable energy interest groups (REI) to state‐level policy‐makers between 1998 and 2010. By means of hazard and tobit regressions, we test the effect of these financial contributions on the probability of RPS adoption and on RPS stringency. The article provides evidence in favor of interest group theory. First, CEI have donated more to state‐level legislators affiliated with the Republican Party than to Democrats while contributions from REI went largely to the latter. Second, there are statistically significant links between the likelihood of RPS adoption and private interest contributions. Contributions from CEI have a negative effect on the likelihood of RPS adoption whereas REI contributions have a positive effect.