Valls finds himself in a devilish position, says Frederic Dabi of polling firm IFOP.
But the alternative is standing his ground and risking that the unions keep up their paralysing unrest through to June 10th, when France will start hosting the Euro 2016 football championships.
"Despite his drop in opinion polls, Valls still has an image of strength, an image of authority -- maybe a bit too much," said Dabi.
Only 26 percent of French voters support the prime minister's tough line, while 69 percent want the labour reform bill withdrawn "to avoid a blockage of the country".
Three months of protests against reforms have led to strikes and blockades at fuel depots and refineries, as well as multiple strikes across the transport sector that could seriously impact the smooth running of Euro 2016 in 10 venues around the country.
"Valls has his back against the wall. If he withdraws the bill, he can't stay in his job," said an MP from the ruling Socialist party, who spoke on condition of anonymity.
The prime minister has been through a rough few months, becoming a hate figure for the more militant left-wing of his party who say the reforms are too liberal and give too many powers to employers over workers.
"How long can he stand firm?" asked the front page of right-wing newspaper Le Figaro on Friday.
It followed an eighth day of protest marches in little more than two months, with between 150,000 and 300,000 people taking to the streets on Thursday. Another is due on June 14th.
When Valls took over as head of the government in March 2014, the Catalan-born prime minister was known for his stormy character and stern demeanour from his stint as interior minister -- not attributes that lend themselves easily to the diplomacy required in the current crisis.
"This country sometimes kills itself with its conservatism, from the impossibility of reform. That's why we are at a crucial moment," Valls told parliament on Thursday.
In a moment of reflection last month to magazine Society, he admitted some "errors" in the "moment and the way" in which the labour reforms were presented.
Valls launched an ambitious attempt to win over sceptics in his party and on the streets with amendments that curved some of the bill's sharper edges, but in the end he had no choice but to force the bill through parliament without a vote.
That brought accusations of being anti-democratic and did nothing to stop the momentum of protests, leading some in the government -- including Finance Minister Michel Sapin -- to start calling for a renegotiation of its most contentious aspects.
"Am I about to withdraw the text? Or rewrite an article that is the very heart of its philosophy?" retorted Valls sardonically.
It marks a sharp dip in fortunes for the prime minister who, for a while last year, was tipped as a likely successor to the unpopular President Francois Hollande, with a presidential election due in May 2017.
But Valls has been leap-frogged as a contender by his own economy minister, Emmanuel Macron, who launched a new political movement last month as a possible first step towards declaring his candidacy.
Having gone this far, polling expert Dabi says Valls has little choice but to push it to the end.
"There are fewer blows to take from keeping the bill, while leaving the door open to modifications and adjustments," said Dabi.
Valls himself says he has no intention of falling on his sword as the labour crisis deepens.
"Just because the CGT (union) is blockading the country does not mean I am about to leave, or that I am going to fold," he said.