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France's Socialists are reeling from humiliating losses in local and European elections this year, with the latter seeing the hard-right National Front top a national vote for the first time.
President Francois Hollande has promised to reverse a rise in unemployment to record highs, helped by tax cuts for companies and lower public spending, but this policy shift is being challenged by the left wing of his Socialist party.
"I called for a sense of responsibility. The left could disappear and France could fall apart such has been the succession of crises - economic crisis, crisis of identity, cultural crisis," Valls said.
"In the face of this, we have to hold strong and over time," he told news channel iTele after a closed-door party meeting.
During the meeting, he also said the left had never been as weak since the creation of the fifth republic in 1958 and warned the National Front's Le Pen could make the run-off in the next presidential vote in 2017, French media reported.
Valls is trying to rally dissident Socialists ahead of crucial votes in parliament on promised tax cuts for business.
One of the leaders of the left wing of the party, meanwhile, said that Hollande should contest a primary in order to be the Socialist candidate for the next election.
"Yes, we will need a primary in 2016," Emmanuel Maurel said in an interview with daily newspaper Le Monde. "Support for a candidate, even if it was Francois Hollande, is not automatic."
Hollande won a Socialist primary ahead of this election in 2012 but no sitting French president has ever gone through a primary in order to seek re-election.
The deeply unpopular Hollande is also facing a strike by railway workers, with the hardline CGT union deciding to extend the protest over a planned rail reform into a fifth day on Sunday.