Hardy began his activity as a revolutionary militant during the Second World War when he joined a tiny Trotskyist group, the Union Communiste, led by Barta (David Korner) a Romanian Trotskyist. Given that the group was clandestine all members adopted cadre names and there was a considerable stress on security within the group. This continues today as does the emphasis of the UCI on orienting towards workers in the workplaces.
The UC did not take part in the regroupment of the other French Trotskyist groups which took place in 1944 and led to the foundation of the Internationalist Communist Party. This was because the UC held that the other Trotskyist groups had not made a balance sheet of what the UC saw as their nationalist deviations in the early period of the war.
The central task of the UC was working around the Renault factory in the Paris area where it had members working and doing educational work in order to develop cadres. In 1947 this work meant that the UC was instrumental in leading the Renault strike which contributed to the fall of the Government. However Hardy was not personally involved in these events due to ill health.
The strain on the UC leading the struggle at Renault and subsequentially the independent SDR union there led to its collapse. After various attempts to revive the UC, a paper, Voix Ouvriere, was launched in 1956 after the Russian invasion of Hungary and the Anglo-French invasion of the Suez canal Zone in Egypt. Among the figures leading this effort were hardy and another former member of the UC Pierre Bois the leading UC militant at Renault. An obscure dispute with Barta seems to have ensured his lack of involvement however.
From 1956 to date hardy has been the central leader of first Voix Ouvriere and after 1968 Lutte Ouvriere and has stamped his character on the group.