The debate between economic and political explanations of the adoption of proportional representation (PR) has yielded mixed results. We reexamine this debate and argue that one has to take the different levels on which the causal mechanisms are located into account. This leads to a novel reformulation of Rokkan’s hypotheses: we claim that PR is introduced when legislators face strong district-level competition and when their parties expect to gain seats from a change of the electoral law. In the empirical part, we model legislators’ support for the PR adoption and evaluate the relative importance of district-level competition and vulnerability resulting from electoral inroads made by Social Democratic candidates; partisan calculations arising from disproportionalities in the allocation of votes to seats; and economic conditions at the district-level, specifically variation in skill profiles. Support for the adoption of PR is explained by a combination of district vulnerabilities and seat-vote disproportionality.