If he wins the primary, Valls will also have to contend with Francois Fillon, a more moderate conservative candidate who advocates a free-market economic platform but shares many of Le Pen's anti-Muslim and anti-immigrant sentiments.
The French left, once the undisputed bedrock of national politics and a principal architect of the European Union, has fallen in esteem to such a degree that winning the presidency in 2017 is almost inconceivable, analysts say. By one poll, the presidential approval rating of Hollande, the Socialist incumbent, has dropped to a minuscule 4 per cent.
As an explanation, analysts generally cite France's stubbornly high unemployment rate and a wave of devastating terrorist attacks that have claimed some 230 lives in the last two years.
In the wake of those attacks, Hollande's Socialist Government has struggled to project an image of strength, responding with measures that critics say are primarily concerned with saving face instead of enacting real change.
In his address, Valls rejected this assumption, insisting with dramatic emphasis that the left still has a viable chance to win the presidency.
"I'm told that the left has no chance, but nothing is written," he said.
"That it will never come together, that the extreme right is qualified for office, but nothing is written. That Fillon is already the next president of the republic, but nothing is written."
He continued: "The left is big and beautiful when it speaks to all the French, when it gathers together, when its destiny is entwined with that of France. France needs the left!"
But the French left is deeply divided within its own ranks, largely thanks to Hollande's controversial proposal to strip French nationality from convicted terrorists who hold dual citizenship. Many on the left insisted that the proposal would enshrine a legal distinction between citizens who are supposed to remain equal before the law, and it was ultimately abandoned. But the broader rift inside the party has remained.
Valls, who has sought to project a certain tough-guy swagger as Prime Minister, has been a constant fixture in that internal struggle, especially vis-a-vis France's Muslim community, the country's largest minority group. During a furore over the "burkini" this past northern summer, for instance, when some 30 French towns banned the modest swimsuit from public beaches, he was among the towns' loudest defenders.
French courts have since overturned many of the burkini bans in the jurisdictions that passed them.
But Valls called the burkini, designed to allow Muslim women to enjoy the beach while respecting traditional dress codes, a "provocation," insisting that the garment was an affront to France's secular values. In response, Muslim leaders quickly accused him of hypocrisy. Supporters of French secularism, they pointed out, do not object that virtually all of France's public holidays are Christian in origin.
Today, Valls suggested that bigger battles lie ahead.
"I note the division on the left," he said, "but until when will we endure this spectacle?"
THE CONTENDERS
FRANCOIS FILLON
The longtime No. 2 of former President Nicolas Sarkozy, this amateur race car driver is now at the wheel of the conservative presidential campaign - representing his Republicans party and its allies from the centre-right. Fillon, 62, presents his platform as "radical:" He wants to cut taxes on business and slash public spending to boost France's stagnant economy and proposes strong measures to reduce immigration "to a minimum".
He also wants loosen the country's stringent labour rules in the hope to encourage hiring, and has a strong focus on traditional family values. He pledged to fight energetically against the Socialist candidate and the far-right.
MARINE LE PEN
Le Pen, 48, is the candidate of her far-right National Front Party. Bolstered by Donald Trump's victory, the election may turn out to be a referendum on her ideas. She's campaigning on an anti-immigration, anti-Islam, nationalist platform and hopes anti-establishment sentiment can propel her to the presidency. That would have repercussions across Europe and for post-war unity. Le Pen wants to lead France out of the European Union and its visa-free zone. Political analysts say she may reach the second round of the presidential election, by coming in the top two in the first round.
MANUEL VALLS
France's Prime Minister since 2014, Valls has jumped into the presidential race only four days after unpopular President Francois Hollande announced he would not run for re-election. Valls, 54, leaves his job to focus on his candidacy in the Socialist primary next month, in which he will be favourite but will face strong competition. A leading yet divisive figure of the Socialist Party, Valls is known for his outspoken, authoritarian style and his tough views on immigration and security. A group of Socialist rebels have criticised his pro-business strategy and called for more left-leaning policies. Among them, former Economy Minister Arnaud Montebourg and Education Minister Benoit Hamon have announced they will compete against him in the Socialist primary.
EMMANUEL MACRON
A former Economy Minister under Francois Hollande, this outspoken and telegenic former investment banker is advocating for pro-free market and a progressive approach to social issues. He notably encouraged startups and passed a law loosening labour rules.
The 38-year-old Macron, who has never held elected office, this year started a political movement called "En Marche!" (In Motion) that he presents as neither right- nor left-wing.
His views have prompted harsh criticism from many members of the Socialist Party who feel that he has betrayed left-wing ideals. Macron has decided not to take part in the Socialist primary.
THE REST
Far-left figure Jean-Luc Melenchon, Greens nominee Yannick Jadot and other independent candidates and smaller parties' leaders are running in the election - as allowed by French law. Anyone can run if they can collect 500 signatures from elected office-holders to support the candidacy.
- AP
