Soft regulatory measures are argued to represent a useful alternative to hard regulation when policy-makers seek to deal with difficult problems. This article investigates the impact of a national guideline on climate and energy planning adopted in Norway, a soft regulatory instrument, which sought to encourage stronger local climate mitigation policies. To assess the effectiveness of this policy approach the research reported here investigated the impact of the guideline on the institutionalization of local climate policy activity in a set of Norwegian municipalities. Our findings indicate that for municipalities in the early phases of developing their climate mitigation policy the planning guideline contributed to legitimizing climate policy by linking it to other policy areas. For municipalities that had come further in developing their climate mitigation policy, however, the planning guideline made little difference. The findings indicate that even if soft policy instruments might be needed in a situation with complex regulatory issues, it is difficult to achieve results without the support of harder policy measures. This is particularly the case if local authorities face financial constraints and lack manpower in their work in developing effective climate mitigation policy.