This research focuses on the politicization of immigration as an issue in local electoral campaigns, comparing the cases of three Italian cities. Based on the idea that immigration must not be understood as a one-dimensional category that parties endorse or dismiss, support or oppose, I investigate its multidimensional nature, and the importance of local factors and opportunities in determining public debates. Focusing on the dimensional choices and framing strategies of competing electoral actors, I propose an account of the different constitutive dimensions of immigration debates, and suggest that parties - next to competing over different issues - also compete with one another by selectively and strategically emphasizing different aspects of the same social reality. In particular, I identify three main dimensions of the immigration issue - the socioeconomic, cultural and religious, and law and order dimension - and seven specific frames corresponding to the arguments and justifications mobilized by political actors to articulate support and opposition to immigration. The construction of public agendas in electoral campaign periods is measured through an empirical content analysis of the coverage of local elections by newspapers and of local parties' electoral manifestos across two campaigns in the cities of Milan, Rome and Prato (2004-2011). The results show not only that debates in different local settings deal with immigration in substantively different ways, but also that parties' electoral strategies rely upon the thematic structure of the issue, exploiting immigration dimensions in order to increase the accessibility and resonance of their messages among local electorates. The results of this dissertation offer one of the first comprehensive analyses of an issue that has too often been considered "emerging" in party competition, showing that when the issue cannot be dismissed, actors compete on its constitutive dimensions by mobilizing aspects on which they enjoy a strategic advantage. These findings pave the way to connect this field of research with other promising areas within the social and political sciences, such as public opinion research and the study of mediatization and communication in party politics, providing new insights into electoral politics and campaigning.