During this period he was known by the codename "Charles the invincible", which referred to his growing alcohol abuse habits and perseverance in seducing women.

In 1953, he created the "Club of the Jacobins", which ideologically was near to the radical (though non-communist) left,[2] and which supported future Prime Minister Pierre Mendès France.

On 2 January 1956 (after Mendès France had lost the Prime Ministry but was still a powerful cabinet figure), Hernu was elected to the French legislature from the 6th sector of the Seine (Aubervilliers, Saint-Denis, Montreuil, Vincennes), on the Republican Front ticket.

In 1996, the magazine L'Express published articles claiming that, under the code names "André" and "Dinu", Hernu had been an agent of the Soviet Union.

According to the newspaper, Admiral Lacoste affirmed that the French spies who planted the bombs acted under the orders of Mitterrand himself (who had died nine years earlier).