The present research identifies a psychological phenomenon that helps to explain how people who prefer the same option to the same degree (e.g., two people equally prefer politician A over politician B) can differ in their negativity toward the same undesired outcome (e.g., one person reacts more negatively toward politician A’s defeat). Across multiple domains and a variety of methodologies (e.g., archival, longitudinal, experimental; N = 12,830), we provide evidence for a prevalent phenomenon we label the opposer’s loss effect. When people frame a preference in terms of opposition to the nonpreferred option (“I’m anti politician B”) versus support for the preferred option (“I’m pro politician A”), it does not change the extremity of their overall preference; however, opposers (vs. supporters) nonetheless report greater negativity to relevant, unwelcome news. As we show, this framing shifts secondary characteristics of the preference, namely, it decreases their feelings of ambivalence in their preference, which amplifies opposers’ negativity when that preference is thwarted. Altogether, these findings advance the literature on framing effects, expand the known antecedents to felt ambivalence, and provide practical advice for forecasting negative, mass sentiment.