Party leader Martine Aubry announced she will seek to become the Socialist candidate for president in the 2012 French election.
Aubry, 60, the daughter of former European Commission President Jacques Delors, took over the party in 2007 after its defeat by the conservative Nicholas Sarkozy, and is currently the mayor of Lille.
I want to restore France's strength, its calm and its unity, Aubry said in Lille, while making her announcement.
I want to give a taste for the future and a desire for a common destiny back to everyone.
She bitterly criticized Sarkozy’s economic policies, accusing him of favoring the the most privileged in France.
The slot became wide open after Dominique Strauss-Kahn, the former favorite, was arrested on rape charges in New York in May.
Aubry’s principal challengers would likely be former party leader Francois Hollande and Segolene Royal, who lost the 2007 election to Sarkozy.
Nominations for the Socialists primaries will close on July 13 and they will vote for the candidate in October.
Sarkozy is widely expected to run again for the conservatives; however current polls indicate he would be easily defeated by either Aibry or Hollande in the first round of voting next April.
Still, left-wing parties in France have been hampered by infighting – the Socialists have not won a national election since 1988 when François Mitterrand was elected president.
Meanwhile, Sarkozy has defended his economic policies in a major press conference, timed to coincide with the Socialists naming new candidates.