SummaryResearch on the relationship between social and political factors is mostly carried out for one election only, both as far as sample survey research as Well as the analysis of aggregate data is concerned. So far little research has been done on the stability over time of relations between social factors and relative party strength.A long term ecological analysis of Danish voting statistics was launched in 1962 at the Institute of Political Science at the University of Aarhus. The aim of this project is to investigate changes over time in the relations between social factors and party strength as well as the effect of social change on the development of the relative strength of Danish political parties. Longitudinal studies of this kind are stimulated by the development of electronic data processing but complicated by the inconsistencies over time in the available census and other statistical data.Results from the elections in the period 1947–1964 are presented for ecological units with varying degree of urbanization, occupational structure and population growth. (1) There is extremely high parallelism in the development of party shares within different ecological units. This pronounced ecological inter‐stability in the development of party strength indicates that there has been little change in the relations between social factors and party strength. Especially short term fluctuations have a high degree of inter‐stability. Long term fluctuations in the relative strength of the Liberal and Conservative party indicate a certain amount of structural change, the Liberal party gaining in strength in urban areas and in this way compensating for losses in the rural parts of the country, the Conservative party gaining both in urban and rural areas. (2) The general stability, which is reflected in the gross results for each party for the country as a whole, to a large extent disguise structural change. Stagnation for the Social Democrats in 1960–1964 was the results of the fact that losses in urban areas were cancelled out by gains in the rural parts of the country. Detailed ecological analysis of electorial data enables us to discover structural changes much earlier than otherwise possible. (3) In spite of an increasing urbanization, industrialization, and decentralization of industry and irrespective of the influence of television, regional political differences are still maintained. Only little egalization can be observed and only in the end of the period in question.It is evident that the period 1947–1964 in many ways is too short for an analysis of the effect of social change on electoral behaviour, especially so for an evaluation of long term fluctuations. Due to planned changes in the administrative structure of the country ‐ the reduction of 1200 municipalities to only 400, it will be difficult to continue the time series presented in this paper. Therefore the immediate extension of the present research program concerns the period 1920–1945. This part of our research will meet the obstacle of inconsistency in the a