French Prime Minister Manuel Valls is set to resign on Tuesday to free himself to do battle for the Socialist nomination in next year's presidential election.
Announcing his candidacy on Monday 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.
His announcement came four days after President Francois Hollande announced he would bow out after a single term, paving the way for Valls to try to become the left's new standard bearer.
"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.
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.
Polls show Marine Le Pen winning or being placed second in the opening round of the election on April 23 but later being defeated by the conservative Francois Fillon.
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.
His government's bleak economic record could also clip his wings.
On Monday, he said he wanted to help the working class "regain its dignity" in the face of globalisation.
He faces a challenge from at least seven other candidates in the two-round primary on January 22 and 29.
Martine Aubry, a veteran Socialist and the mayor of the northern city of Lille, said on Monday it was "not a given" that she would back him.
Polls show Valls nonetheless winning the nomination but trailing in the presidential race, dragged down by competition on the left from business-friendly former economy minister Emmanuel Macron, an independent, and the fiery hard-left Jean-Luc Melenchon of the Left Party.
When Valls last sought the Socialist nomination five years ago, he garnered only 5.6 percent.
He went on to become a spokesman for Hollande's campaign and was later rewarded with the interior portfolio.
In 2014, he was promoted to premier with a mandate to rein in a group of unruly ministers who were undermining Hollande's authority.
Within months he had a rival for the title of reformer-in-chief, in the telegenic banker-turned-economy minister Macron, a protege of Hollande.
In August, Macron walked out on his boss to further his own leadership ambitions while Valls remained by Hollande's side.
In recent weeks, however, Valls had made clear he saw himself -- and not Hollande -- as the Socialists' best chance of keeping the keys to the Elysee Palace.