The design of campaigns for the improvement of intergroup attitudes requires innovative approaches that consider both the characteristics of the messages and the psychological processes they evoke. This work addresses the study of factors that increase the persuasive effectiveness of testimonial messages aimed at improving attitudes towards stigmatized immigrants. An experiment was conducted using a representative sample of 443 participants of Spanish origin on the effect of similarity to the protagonist and the narrative voice. Two mediating mechanisms (identification with the protagonist and cognitive elaboration) were evaluated, and the indirect effect of the two independent variables was studied with respect to two dependent variables: the attitude towards immigration and the intention to collaborate with NGOs to support immigrants. Similarity to the protagonist of the narrative message increased identification only when the participants read the version written in the first person. In addition, a conditional process model was tested, revealing that identification increased cognitive elaboration, which, in turn, was associated with a more favorable attitude towards immigration and a greater intention to collaborate with immigrant support organizations. This study highlights the relevance of the characteristics of narrative messages to increase affective (identification) and cognitive (elaboration) processes that explain their persuasive impact. The results are discussed in the context of research on narrative persuasion and the design of campaigns for the prevention of racism and xenophobia.