ABSTRACT Sudden crises present a ‘stress test’ for democratic policymaking. Amidst global crises the capacity for swift decisions is becoming increasingly important but reacting to abrupt challenges also easily leads to violations of democratic norms, as recent studies on Covid-19 pandemic demonstrate. To advance our understanding of this pressing tension that thus far has been mainly approached via broad regime-level comparisons, this study examines formal and informal coordinative institutions that facilitate efficient and democratic policy response in crisis situations. Applying the core executive model, we focus on Finland’s decision to apply for NATO membership that unfolded rapidly after Russia attacked Ukraine. Through detailed process tracing analysis, we show how the country’s strongly entrenched doctrine of military non-alignment was overturned with broad cross-party consensus in under three months via intensive mediation involving all key actors and governing institutions. Adding important nuance to constitutionally centred studies of crisis decision-making, our analysis shows how strongly institutionalised coordination mechanisms can facilitate radical policy changes to status quo even in the context of dispersed policymaking resources and high partisan fragmentation. Our findings especially underline the complementary roles and interplay of formal rules and informal practices, showing that in crisis situations the latter acquire fundamental importance.