This paper argues that the effectiveness of fiscal policy may increase markedly during periods of low nominal interest rates and high, persistent, unemployment. An increase in government spending boosts economic activity and reduces the unemployment rate both in the present and in the future. As a less disconcerting future spurs a rise in private consumption, unemployment falls even further and triggers an additional rise in private demand, and so on. In a stylized model, I show that the marginal impact of government spending on output is equal to the reciprocal of the elasticity of intertemporal substitution. In a more realistic framework, the effect is somewhat attenuated and displays significant nonlinearities with respect to the depth of the crisis as well as the size of the stimulus package. But in a severe recession with an unemployment rate of eight percent or above, the fiscal multiplier is equal to 1.5.