The losers' consent principle—that in democracies losers must accept and comply with electoral defeat—appears increasingly challenged. This erosion has been linked to affective polarization, yet the precise connection between the two phenomena remains underspecified. We advance and test a novel theoretical framework, arguing that animosity toward the election winner—the party expected to control executive power—motivates losers to contest results due to anticipated harmful government actions. Using an original pre-registered experiment fielded during Austria's 2024 elections and a regression-discontinuity design with global data, we show that losing to one's most disliked party lowers democratic satisfaction and raises support for contestation. Mixed-methods analyses reveal these reactions stem from substantive concerns about policy and undemocratic actions by the winner, not mere emotion. Our findings clarify a pathway from affective polarization to losers' dissent, via winner animosity, and raise normative questions about the role of consent in democracy when contestation reflects militant democracy rationales.