Grévy was elected vice-president of the National Assembly in April 1849. The same month he protested against the president's decision to launch an expedition against the revolutionary Roman Republic, a liberal state created during the Italian War of Independence, but the invasion proceeded on and ultimately succeeded in restoring Papal rule. In 1851, his fear that Louis-Napoléon intended to perpetuate himself in power was proven true, when the president seized dictatorial power with a self-coup on 2 December, in which Grévy was arrested and imprisoned in Mazas Prison. He was released shortly after but retired from politics in the subsequent French Empire, under now emperor Napoleon III, and returned to his law practice.