Elite and mass level politics in many Western democracies is increasingly characterised by the expression of negative feelings towards political out-groups. While the existence of these feelings is well-documented, there is little evidence on the consequences of activating political identities during election campaigns. We test whether fundraising emails containing negative or positive political identity cues lead party supporters to donate money via a large pre-registered digital field experiment conducted in collaboration with a British political party. We find that emails containing negative as opposed to positive identity cues lead to a higher number and frequency of donations. We also find that negative identity cues were only effective when paired with an issue identity rather than a traditional party identity cue, resulting in a 15% increase in the probability of donating over the untreated control. Our results provide novel experimental evidence on the behavioural effects of activating identities in real-world political campaigns.