Metaphors have been shown to be effective explanatory and communicative tools, shaping how people think and reason about complex domains. To date, however, most studies have addressed only coarse-grained effects of metaphor framing, leaving many questions unanswered about the relative power of metaphor compared to more literal linguistic framing devices. We addressed this issue in a large, pre-registered framing study, comparing the effects of describing the role of police officers as (a) metaphorical guardians of a community (b) literal protectors of a community, and (c) a no-label control. We found no main effect of framing condition, suggesting that positively valenced metaphors may exert little influence on their own in this domain. However, we did observe an interaction between condition and political ideology, such that the guardian metaphor was especially effective at improving attitudes towards police officers for liberals, whose initial approval ratings were relatively low.