It is still unclear why the transition temperature Tc of cuprate superconductors falls with underdoping. The doping dependence of the critical magnetic field Hc2 is directly relevant to this question, but different estimates of Hc2 are in sharp contradiction. We resolve this contradiction by tracking the characteristic field scale of superconducting fluctuations as a function of doping, via measurements of the Nernst effect in La1.8−xEu0.2SrxCuO4. Hc2 is found to fall with underdoping, with a minimum where stripe order is strong. The same non-monotonic behaviour is observed in the archetypal cuprate superconductor YBa2Cu3Oy. We conclude that competing states such as stripe order weaken superconductivity and cause both Hc2 and Tc to fall as cuprates become underdoped.