Negative partisanship, voters’ rejection of a party, increasingly garners scholarly attention. Yet we lack robust empirical evidence on the nature of the concept and how it shapes attitudes towards citizens of the “other side” of politics. In this paper I argue negative partisanship should be conceptualised as a type of social identity, beyond a mere “dislike” of a party. Leveraging a three-wave online panel administered during the 2021 German federal elections, I test the measurement properties of a multi-item scale measuring negative party identification. Further, I examine the effects of negative identities on attitudes towards other citizens by combining the online panel with a nine-country survey. I find that negative identification is at least as stable as (positive) partisanship and it predicts hostile attitudes towards out-voters. These findings suggest that, when dislike of a party becomes part of one’s sense of self, the consequences for social cohesion are particularly egregious.