Can campaign strategies persuade voters to support non-coethnic candidates in multiethnic societies? While the polarized nature of politics in ethnically divided electorates often leads candidates to mobilize support by activating latent ethnic identities, candidates also often seek to compete for national office by reaching out to voters across ethnic cleavages. In the context of Africa’s oft-ethnicized elections, we argue that candidates can inadvertently dampen their political support when campaigning among non-coethnic voters. Drawing on the logic of group threat, we claim that the physical presence of a candidate in a non-coethnic constituency can serve to heighten voters’ sense of intergroup competition. To corroborate this expectation, we are able to leverage a natural experiment during the 2017 Kenyan presidential elections. We exploit the timing of a locally-representative survey that straddled an unexpected campaign rally held by a leading opposition presidential candidate in a constituency largely populated by the coethnics of his rival, the incumbent president. We show that the rally backfired politically. Rather than increase support for the opposition candidate, the rally significantly decreased the candidate’s favorability among local non-coethnic voters. Moreover, we find that the candidate’s rally ultimately primed those voters to think of their own ethnic identities—a finding supported by the sharp decline in the proportion of respondents identifying in national rather than ethnic terms.