As political power tends to be wielded in the form of voting power in the national assembly, especially under the institutions of party government, one needs a method to calculate the voting power of political parties, both longitudinally and for a cross-section of European democracies. This paper suggests such a method, derived from the power index approach in cooperative game theory. The application of the method on the history of democracy in the two German nations results in party scores—mandates, ex ante and ex post voting power—that are much in congruence with the standard interpretations of the interwar period and the post-war politics in these two countries.