Hours after he fell short of making it into the final phase of France’s presidential election, the veteran leftist Jean-Luc Mélenchon had a message for his supporters: they should not give “a single vote” to far-right candidate Marine Le Pen in the second round in two weeks. Now the identity of France’s next president might depend on whether they listen.

Mélenchon’s strong showing in Sunday’s first round, when he won 22 per cent of the vote, has put him and his voters in the position of kingmakers as incumbent president Emmanuel Macron battles Le Pen during the final days of campaigning. Macron, in particular, needs as many Mélenchon supporters as possible to back him to win on April 24.

Mélenchon’s message of rejection for Le Pen would seem to favour the president. But Mélenchon stopped short of prompting supporters to vote Macron and his party is due to consult members on whether to do so. The president faces a fight to win over far-left supporters who are far less inclined to help him than in 2017, when he coasted to victory against Le Pen, and stop them from abstaining.

In 2017 Macron won backing from about 50 per cent of Mélenchon’s base in the second round, surveys showed.

“I voted for Macron five years ago and for five years I bitterly regretted it,” said Marion Boué-Arbieu, 29, who joined an impromptu street party on Sunday night outside Mélenchon’s Paris headquarters, after her favoured candidate won strong support in cities from Strasbourg to Lille and Marseille.

A sports teacher in Aubervilliers, a poorer area on the outskirts of Paris, Boué-Arbieu said she was strongly tempted to cast a blank vote and was critical of Macron’s social record, saying growing numbers of people were requesting help and meals at a social centre where she volunteers.

“No way I’ll vote for Marine Le Pen but will Macron just make things worse for poorer families? I don’t know what I’ll do,” said Boué-Arbieu.

As Macron rolled out his second round campaign on Monday with a visit to northern France, he made a deliberate attempt to emphasise his social manifesto but the two politicians are diametrically opposed on many policies. The leftwinger wants to roll back the retirement age, leave Nato and get France out of nuclear energy.

Macron, a former banker who came to power on a pro-business, pro-Europe platform, is so vehemently opposed by some Mélenchon supporters — who see him as favouring the rich — that up to one-third are inclined to abstain in the second round, with another third considering voting for Le Pen, polls have showed.

Despite Mélenchon’s message not to support Le Pen, some of his core working class voters share views that overlap with hers on matters such as the cost of living and a more protectionist vision of France. Le Pen has also made overtures by saying she could appoint leftwingers in her government if she is elected.

Without backing from a deep enough pool of Mélenchon voters, including some who voted tactically in the first round as they saw him rise in polls, Macron will be in uncomfortable territory for the second round. Polls already put him at roughly 52 per cent versus Le Pen’s 48 per cent.

“Macron barely has any more votes to draw on when it comes to the right,” said Dominique Reynié, a political scientist at Sciences Po university, pointing to the unexpectedly low support for the centre-right Les Républicains — Macron’s other main second-round bedfellows.

Ultimately, a deep attachment to Europe and the euro among many French voters, including Mélenchon’s, may help Macron, but only just, Reynié said. “He needs to get a lot of Mélenchon voters, otherwise I’m not sure how he’s going to make it.”

Mélenchon’s strong showing alongside Le Pen’s underscores an upheaval in French politics that has relegated France’s longstanding parties of government — the Socialists and Les Républicains — into political obscurity. Until Macron, they had provided every president since the 1950s. On Sunday, each scored less than 5 per cent.

In the longer run, this spells a possible redrawing of party lines on the left, particularly if 70-year-old Mélenchon steps aside as expected after his third run for the Elysée.

Another casualty of this broader shake-up has been the red line that once divided the far-right from other parties. Macron cannot count on parties to club together to block its rise in a so-called “republican front”, as happened in the past.

The pool of votes Macron could tap into was “no longer in one party or another, or in the right or the left, they have disappeared”, finance minister Bruno Le Maire told RTL radio on Monday. He appealed instead to “the soul of the French.” 

As the second-round campaign cranks up, Macron supporters are also trying to capitalise on Mélenchon’s appeal to young, environmentally-conscious voters, highlighting Le Pen’s manifesto — her central measure is to remove wind turbines — and emphasising their own inclination to pursue climate-friendly goals.

But Macron allies said he had no plans to make significant changes to his economic proposals, such as a contentious plan bid to raise the retirement age from 62 to 65.

“It doesn’t work like that, by putting band-aids on,” Macron’s Europe minister Clément Beaune said on Sunday night. “There are major social measures in our plans. We need to explain them to people . . . who voted on the left.” 

Many have yet to be convinced. Guillaume Godin, a 39-year-old teacher who voted for Mélenchon, said Macron and Le Pen were as bad as each other on environmental issues. “I don’t feel like going out to help Macron again,” Godin said. “It’ll be another lost five years either way.”

Additional reporting by Leila Abboud and Victor Mallet