During the 2019 Indonesian presidential election, two supporter groups of presidential candidates exhibited negative attitudes toward each other. This study aimed to explain the mediating effect of collective narcissism on the relationship between three predictors (religiosity, group-based anger, and shared reality) and attitudes toward opposing groups. We conducted two surveys before and during the official campaign period (Study 1, n = 300) and after the official campaign period until one day after the election (Study 2, n = 158). The results of Study 1 showed that group-based anger and shared reality had a significant relationship with negative attitudes toward the opposing group. In Study 2, collective narcissism was found to significantly predict negative attitudes toward the opposing group and mediate the relationship between shared reality and negative attitudes. We also found a mediating effect of collective narcissism on the relationship between group-based anger and negative attitudes. These findings indicate that the strength of collective narcissism in the ingroup can predict negative attitudes toward the opposing group. © 2025 Society for the Psychological Study of Social Issues.