Democrats and Republicans clearly dislike one another. Yet, scholars debate whether policy disagreement or partisan identity, per se, drives interparty animus. Past studies suggest the relationship between partisanship and interpersonal affect is spurious, driven by inferred policy preferences. We argue, instead, that policy preferences signal partisan identity when the parties’ stances on an issue are well-known. Using a nationally representative survey and four preregistered experiments, we disentangle the effects of policy disagreement and partisan identity on interpersonal affect. Our findings suggest that partisan identity is the principal mechanism of affective polarization, and that policy preferences factor into affective polarization largely by signaling partisan identity. However, our results also affirm that policy disagreement in itself drives interpersonal affect. This provides evidence that partisanship reflects an emotional attachment to a political party, not merely a running tally of rational considerations.