In May 1815 he entered the Chamber of Representatives and at once took an active part in the debates as a member of the Liberal Opposition, and strenuously opposed the election of the son of Napoleon as emperor after his father's abdication.

He defended with great intrepidity the principal political victims of the reaction, among others, in conjunction with Pierre-Nicolas Berryer [fr], Marshal Ney; and in October 1815 boldly published a tractate entitled Libre Defense des accusés.

In 1827, he was elected a member of the Chamber of Deputies and in 1830 he voted the address of the 221, and on 28 February[clarification needed] he was in the streets exhorting the citizens to resistance.

After the 1848 abdication of Louis Philippe, Dupin introduced the young Count of Paris into the chamber, and proposed him as king with his mother as regent.

[1] This attempt failed, but Dupin submitted to circumstances, and, retaining the office of procureur général, his first act was to decide that justice should henceforth be rendered to the "name of the French people."