Elaborating on earlier work we develop a theoretical model of opinion formation and electoral choice based on individual variables (degree of political sophistication, ideological orientation, party identification, timing of the individual decision) and contextual variables (intensity of the electoral campaign). We test this model with data from the 1999 Swiss election panel study. The three-wave panel carried out in three cantons allows for a comparative analysis of campaign effects on opinion

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... formation. Empirical results provide strong support for our assumption that electoral campaigns matter only for a specific category of voters, namely those who hold no party identification and who make their decision during the campaign. By contrast, irrespective of their political sophistication and other individual characteristics, voters with party identification and "early deciders" are hardly influenced by campaign activities. Also in line with our theoretical expectations, these findings hold when the electoral campaign is moderately intensive. Changes towards highly intensive, respectively towards very little intensive, electoral campaigns are, however, likely to modify the picture. 1 This paper presents preliminary results of a research project on the 1999 Swiss elections. We thank the Swiss Science Foundation for its financial support (subsidy 5004-056086). We thank Simon Hug for his comments.