Macron, a 39-year-old former economy minister, was given little chance when he launched a new political movement "En Marche" (On the Move) last year ahead of a vote that was billed as a fight between conservatives and the far-right.
But the pro-European progressive is now a frontrunner to become France's next leader and will draw strength from an alliance with veteran centrist Francois Bayrou, who decided against mounting a rival presidential bid on Wednesday.
The two met on Thursday, with Macron hoping the tie-up will boost his chances after a tricky 10 days that have seen him lose momentum just as far-right candidate Marine Le Pen picks up speed.
Explaining his decision to join forces with Macron, Bayrou said Thursday the two shared a rejection of the traditional French left-right political system.
"We just had the feeling that we could not go on as before," he said, adding that it was "no secret" that the two had had differences in the past but they had put them behind them.
Francois Bayrou, left, and Emmanuel Macron, pose for photographers after their meeting on Thursday in Paris. Source: AAP
The 65-year-old veteran who finished third in the 2007 presidential election said: "My role is very simple -- I am here to help."
Macron said their alliance was necessary at a time when France faces "a deep moral crisis".
Expect the unexpected
While commentators said Bayrou gave Macron's campaign more substance, a poll by Tilder and OpinionWay for LCI television showed that 72 percent of voters believe the tie-up "changes nothing".
Macron's unforeseen rise illustrates the difficulty in forecasting this year's two-stage election on April 23 and May 7 which is being widely watched by governments and investors around the world.
"It might be that we only see what's going to happen on the night of the election," veteran French political watcher Philippe Braud of Sciences Po university, told AFP on Thursday.
Two new polls on Thursday showed anti-EU far-right leader Le Pen stretching her lead in the first round, where she would win 25-27.5 percent if it were held now.
Le Pen, 48, whose foreign policy speech Thursday was disrupted by a topless protester from the Femen feminist group, is still forecast to lose in the second round -- but she appears to be closing the gap.
Far-right leader, Marine Le Pen, has stretched her lead in the first round of the French presidential elections. Source: AAP
The most likely winner is therefore either Macron or Francois Fillon, the long-time favourite and conservative candidate for the right-wing Republicans party.
But the unstable international background -- from Donald Trump and Brexit to the surge of right-wing nationalism -- is mirrored by an anti-establishment and angry mood in France.
Unpopular Socialist President Francois Hollande decided not to run for re-election in December after a five-year term marked by terror attacks and stubbornly high unemployment.
Both the Republicans and the Socialist parties discarded the most obvious candidates when choosing their nominee in primary votes.
And Le Pen and Fillon both face investigations into their use of allegedly fake parliamentary aides.
Fillon was described as "completely lost" by former right-wing president Nicolas Sarkozy after they lunched together last week, according to the Canard Enchaine newspaper.
French conservative Francois Fillon Source: AAP
A final election twist, largely overlooked until recently, is the potential for a tie-up between the splintered leftist candidates which could produce another upset.
Polls show that Socialist candidate Benoit Hamon, Communist-backed Jean-Luc Melenchon and environmentalist Yannick Jadot have enough supporters to mount a serious challenge together.
"Benoit Hamon has reached out to Jean-Luc Melenchon, he's even reached out to Yannick Jadot," the spokesman for the Socialist government, Stephane Le Foll, said Thursday.
Le Foll urged Hamon to make a deal quickly, adding: "Personally I'm in favour of alliances."
France's Socialists are still haunted by the 2002 presidential election when their divisions led them to be knocked out in the first round, with far-right leader Jean-Marie Le Pen -- Marine's father -- going through to the run-off.
Colonial gaffe
A leftist alliance would make Macron's route to the presidency more difficult, but he can still claim to be the only centrist in the race.
His platform -- set to be announced fully on March 2 -- is more pro-business and reform-minded than his leftist rivals who have large tax-and-spend programmes.
He is also instinctively pro-European and at ease with multiculturalism in France, whereas Fillon and Le Pen have railed against the threat to French identity posed by Muslims in particular.
But the ex-banker remains inexperienced, having served only two years as an economy minister and the same amount of time as an advisor to Hollande.
