How do individuals react to fluctuations in the reliability of mutual defence within NATO? The toss-up election between Donald Trump and Kamala Harris for President of the United States featured two candidates with diametrically-opposed stances towards NATO. On the campaign trail, Trump insinuated that he would encourage Russian attacks on NATO allies, provided the targeted states do not meet their NATO spending commitments. For her part, Harris promised a business-as-usual approach to NATO and Article 5 commitments. The uncertainty of the outcome just one week before election day presented a rare opportunity to compare the effects of hegemonic withdrawal and its counterfactual on public attitudes towards defence. Using a survey experiment in the United Kingdom, Sweden, and Germany, we explore how the threat of hegemonic withdrawal influences public willingness in NATO countries to, on the one hand, increase defence spending, and on the other, come to an ally's defence in the event of an attack. Respondents were randomly exposed to a vignette predicting either a Trump or Harris victory, alongside the respective candidate's policy positions towards the alliance. We then exposed respondents to a second vignette in which a NATO member is under attack, this time priming either in-group or out-group sentiment between the respondent's country and the allied country in need. Despite the universal promise of mutual defence under Article 5, we hypothesize that fissures in alliance cohesion will appear when solidarity must be extended to an ally that deviates from alliance norms.