Elected mayor of Conflans-Sainte-Honorine in 1977, he led the opposition to Mitterrand inside the Socialist Party (as a candidate of the right wing of the party). After the defeat of the left at the 1978 legislative election, he tried to take over the leadership of the party. In spite of his alliance with Pierre Mauroy, the number 2 of the PS, he lost at the Metz Congress (1979). As the Socialist Party's most popular politician at the time (including Mitterrand himself), he announced that he would run for president; but his "Call of Conflans" did not result in majority support within the PS, and he withdrew his candidacy. Mitterrand was the successful Socialist candidate in the 1981 presidential election.