This paper proposes that variations in partisanship and the partisan vote are determined by ideological conflicts and the style of political competition, the latter being defined chiefly by the electoral system applied. It proceeds in three steps. First, it reviews the conventional modernisation-based and thus essentially sociological explanation of variations in partisanship and the partisan vote and advances a “political” complement to it. Then, it reviews the development of partisanship and the partisan vote in six West European democracies and in the US. Third and finally, it puts the “political” explanation of partisanship to a preliminary test – this however by analysing a still incomplete data set. The results, preliminary as they are, suggest that the electoral system applied, but also the degree of politicisation of the electorate at a given election determines partisanship and the partisan vote much stronger than socio-political modernisation does.