Recent discussions of Swedish political change have focused on the decline of Social Democratic ‘hegemony’ and on the end of the ‘Swedish model’. In contrast to preference– or interest–driven explanations for these developments, this paper investigates the impact of constitutional changes made in 1969 in Sweden, which included the elimination of the Upper House or First Chamber of the Swedish parliament and the introduction of a more directly proportional electoral system. Using a simulation model, the actual electoral results from 1969 through 1994 were plugged into the formulas set forth by the old constitutional rules, in order to generate the number of parliamentary seats each party would have received under the old system. This simulation shows that the Social Democratic Party would have received a significantly larger share of parliamentary seats under the old constitutional rules than under the current constitution. Thus one can conclude that the new constitution decreased Social Democratic power in Sweden.