A new poll on Monday showed Valls losing momentum as the primary votes on January 22 and 29 draw closer
After shunting aside his long-time boss, deeply unpopular President Francois Hollande, Valls was hoping for a smooth run to the nomination in the Socialist primary vote later this month.
But since Hollande decided not to run for re-election in December, the 54-year-old Spanish-born centrist has fallen from clear favourite to a man with a serious political fight on his hands.
A new poll on Monday showed Valls losing momentum as the primary votes on January 22 and 29 draw closer, with his leftist rival and ex-ministerial colleague Arnaud Montebourg beating him in some scenarios.
"I'm trying to embody a reformist leftwing which rejects liberalism but tries to make progress at the same time," Valls said in an interview with France 2 television last week.
This was reflected in his programme, unveiled to little fanfare on January 3, that contained few of his formerly bold reform ideas.
The father-of-four came to national prominence as a tough-talker who wanted to modernise his party by shrinking state spending, helping businesses and extending working hours.
His record as prime minister for two and half years under Hollande includes pushing through liberalising economic policies considered too rightwing by many Socialists.
But now he needs the support of leftwing grassroots voters, many of whom prefer the traditional big state Socialist agenda of Montebourg, a former industry minister, or Benoit Hamon, an ex-education minister.
"I know that lots of people liked me when I was a taboo-breaker. At five percent (in the polls)! But of course I've changed. I haven't changed my convictions but I've grown wiser," Valls said last week.
Only a few hundred people turned out to a Valls event at the weekend in northern France.
"He's Dr Jekyll and Mr Hyde," joked the green candidate in the presidential election, Yannick Jadot, on Monday.
In focus particularly is his declaration that he wants to phase out executive powers that allow the prime minister to force legislation through parliament -- having used it six times while in office.
An aide in Hollande's camp, quoted by Le Monde newspaper this week, was even more biting in his assessment of Valls' campaign, which has already changed its slogan once.
The president has apparently concluded that Valls does not have a project. "'His project was to get rid of me' is what he says," the aide was reported as saying on Monday.
The same criticism was levelled at Hollande this week by a former speechwriter who accused the head of state of wanting to become president but having no idea what to do with the power once in office.
Valls is still hoping that the party will not take the path of Britain's Labour party, which has lurched to the left since Jeremy Corbyn's election in 2015.
Strong performances in three televised debates in the next fortnight could help change the momentum of the contest.
But even if he does make it through the primary contest, a far tougher test awaits Valls in the first round of voting in the presidential election in April.
Polls currently forecast the Socialist party candidate to finish fourth or fifth, behind two other former Socialists who are running as independents, the centrist Emmanuel Macron and the hard-left Jean-Luc Melenchon.
Surveys currently tip rightwing Republicans candidate Francois Fillon to win and face far-right nominee Marine Le Pen in the second-round run-off in May, though analysts caution against making predictions.
With the full range of candidates still unknown and politics in Western nations delivering frequent upsets, the race remains open and could well tighten in the months ahead.