The origins and implications of partisan identification are well-studied, but negative partisan attitudes—dislike for a particular party—have escaped such scrutiny, even as the politics of negativity enjoys sustained popularity, especially come election time. In this paper we build upon the comparatively modest negative partisanship literature to consider the effects of negative partisan attitudes on a range of political behaviours. There are reasons to suspect that negative and positive partisanship may have different effects; thus, accounting for the unique influence of negative attitudes is important for understanding the full effect of partisanship on political behaviour. Our results, based upon Canadian Election Study data from 2008 and 2011, reveal that, in addition to vote choice, negative partisanship influences voter turnout and a range of political activities, both related and unrelated to parties. These findings provide evidence of the power of the “dark side” of partisanship.