Although opinion polls bill Valls as the favorite for the Socialists' ticket, they also forecast that neither he nor any other left-wing candidate will win the election, rather that conservative candidate Francois Fillon will beat far-right National Front leader Marine Le Pen in a run-off.
"We're told the left does not stand a chance, but nothing's set in stone," Valls said to applause from supporters in his fiefdom of Evry, the gritty southern suburb of Paris where he was mayor for over a decade.
"I want us to lead the left to victory!" the 54-year-old said, urging his much-divided camp to unite.
The path was cleared for Valls to enter the race last week when deeply unpopular President Francois Hollande announced he would be the first leader since France's Fifth Republic was created in 1958 not to seek a second mandate.
Valls, 54, is a law-and-order hardliner whose business-friendly economic stance face attack from his rivals from the traditional left of the party in campaigning ahead of the Socialist-led left-wing primaries in late January.
Valls will also have to extricate himself from Hollande's turbulent five years at the helm of the euro zone's number two economy if he is to persuade voters he is the best candidate to heal the party's rifts.
"We must unite: My candidacy is one of conciliation, of reconciliation," he said.