Announcing his candidacy yesterday France's prime minister of the past two-and-a-half years said he would step down to try to rally the fractured left ahead of a primary in January.
Health Minister Marisol Touraine and Interior Minister Bernard Cazeneuve are among those tipped to succeed him as premier for the six months until the next legislative elections.
In a combative speech, 54-year-old Valls vowed to take the fight to the conservative opposition and the far-right National Front, who are both leading the Socialists in election polls.
"My candidacy is one of reconciliation," Valls, seen as a divisive figure, said in a speech from his political base in the tough Paris suburb of Evry.
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He warned of the risk of far-right leader Marine Le Pen pulling off a repeat of France's 2002 electoral upset when her father Jean-Marie Le Pen edged the Socialist candidate for a place in the presidential runoff.
Le Pen's nationalist policies would "ruin the working class", he said.
Valls attacked the Thatcherite Fillon, accusing him of rehashing "the old recipes of the 1980s" by promising to cut spending and social programmes.
"We're told that Francois Fillon is the next president of the Republic. Nothing is set in stone," he said defiantly.
Valls faces an uphill task to unite the divided French left, reeling after four tumultuous years under Hollande.
The prime minister himself is a polarising figure who has turned off many lifelong Socialists by using decrees to force contested labour reforms through parliament and endorsing controversial bans last summer on the Islamic "burkini" swimsuit.
Yesterday, he said he wanted to help the working class "regain its dignity" in the face of globalisation.