We investigate the antecedents of affective polarization in the American public, and focus specifically on the driving role of exposure to darker forms of campaign communication (negativity, incivility, populist rhetoric) and the intervening role of individual populist attitudes. Experimental evidence was gathered among a sample of US respondents (MTurk, N = 1,081); respondents were randomly exposed to a campaign message from a fictive candidate framed either positively or negatively, and afterwards asked to express their attitudes towards Democrats and Republicans. Results show that exposure to harsher forms of campaign negativity (character attacks associated with political incivility and populist messages) drives affective polarization upwards when compared to exposure to positive messages. We also show both a direct and moderating effect of populist attitudes: populist individuals are more likely to “like” negative campaign messages (they find them more amusing and fairer) and report higher levels of affective polarization. Furthermore, exposure to negative messages is associated with greater affective polarization particularly among respondents high in populist attitudes.