Cedric Villani, defied the ruling party and announced his candidacy for the Paris mayorship. A poll puts him neck and neck with Macron's official candidate, Benjamin Griveaux.
In early September, French children returned to school and lawmakers to the hallowed halls of the Assemblée Nationale.
But for French President Emmanuel Macron, who was starting the second half of his term following a summer spent flexing his diplomatic muscles, the "rentrée" was quickly soured.
In an act of defiance, One of La Republique En Marche (LREM) most recognisable MPs, Cedric Villani, announced his candidacy for the Paris mayorship, exposing the growing pains of Macron's ruling party and endangering its chances of winning the highly-symbolic Parisian town hall.
A poll released on Sunday show the two LREM candidates neck-and-neck behind Anne Hidalgo, the incumbent mayor from the Socialist Party.
LREM endorsed Benjamin Griveaux, a former government spokesperson and an MP for Paris, as its official candidate for the March 2020 mayoral election in the French capital on July 10.
But the appointment process was decried as "flawed" by Villani, a fellow LREM MP and world-famous mathematician — he was awarded the Fields Medal, dubbed the Nobel Prize for mathematics, in 2010.
Griveaux proposed that Villani "co-pilot" his campaign but his colleague chose instead to let rumours swirl around Paris the whole summer before officially launching his own bid on September 4.
"LREM is strongly united by its support to Emmanuel Macron on such topics as Europe and on the idea that reforms that no one undertook before must now be taken," Bruno Cautres, a Political Sciences researcher at Sciences Po, told Euronews.
"But it is now discovering that like any other political party, it must now manage a diversity of thoughts and pluralities of opinions," he added.
The party was launched just three years ago by Macron as he sought to upend the political establishment to rise to the highest office in the country and aimed to attract moderate right- and left-wing voters under a pro-reform, pro-European agenda.
Macron's bet paid off and just a month after his ascension to the Elysée Palace, LREM stormed to the Assemblée Nationale, winning 314 out of 577 seats and relegating the traditional Socialist and Les Republicains parties into a — weak — opposition.
But cracks are seemingly starting to appear in the majority, which has sometimes been described by commentators and opponents as too docile. In March, 50 LREM MPs abstained from a vote on an anti-hooligan bill they deemed too tough, one of them even quitting the party in protest.
Another MP was dismissed in June over comments she had made on a draft bill to extend medically assisted procreation rights.
Villani is just the latest crack but despite his dissidence, he was not similarly dismissed.
The eccentric mathematician, who until very recently sported shoulder-length hair and was never seen without an ascot necktie and/or a large spider brooch, argued that his candidacy was faithful to LREM's DNA.
But other reasons may be at play.
"(LREM) continues to affirm that it aims to do politics differently than legacy parties," Cautres said.
"Excluding the Fields Medal for mathematics, for a party that prizes expertise, competence and education, would be a serious communication mistake.
"There's also a tactical reason. Let's imagine Cedric Villani is elected mayor of Paris, it would have been a great tactical mistake to exclude him so LREM is being pragmatic in its positioning," he added.
However, he said: "We can assume that his notoriety, and his general profile, are not unrelated to this decision...and it's true that this may set a precedent as it will be difficult now to exclude a dissenting candidate in a municipal election."
For now, though, neither of the two candidates are leading in the polls. A survey commissioned by Sud Radio and Le Journal du Dimanche, released by Ifop on Sunday puts them in close second and third positions.
Hidalgo, the incumbent, is projected to win 24% of votes in the first round. Griveaux would come in second with 17% and Villani, third, with 15%.
"There's an effect of division because, since the announcement of Villani's candidacy, voting intention for Griveaux has fallen," Cautres flagged, stressing however that "electoral sociology is more complicated than that."
"It may well be that Villani has also captured votes from Hidalgo as well," he said.
The French two-round electoral system also prompts most voters to cast their ballot for their preferred candidate in the first round and to then vote "intelligently" in the runoff —meaning that they choose the one closest to their first choice or most likely to beat their least-favourite candidate.
So should Villani or Griveaux make it into the runoff, they could be boosted by the other's votes. Or, voters could instead flock to other candidates.
All will depend on their programmes but it's difficult to see how two candidates from the same political party will be able to set themselves apart from one another.
"For now we know very little," Cautres said.
Villani has indicated the environment would be central to his programme and that "he wants to be faithful to the LREM spirit. "But what does it mean in this campaign to remain faithful to the party's DNA?" he added.
"The game is on."
Playing with matches
Macron has largely stayed out of the Paris debacle but he is keen to close ranks as the second half of his term starts.
The first half was significantly derailed by the Gilets Jaunes crisis and his margin of manoeuvre to pass controversial reforms is tight as opponents will soon start building their campaign for the 2022 presidential ballot.
On his agenda are reforms to the pension system, the economy, the country's welfare state and immigration policy. On the last point, to ensure his lawmakers keep to the official line, he was set to brief them on Monday evening.
Of particular concern are his MPs identifying as centre-left — the same who abstained from the anti-hooligan bill.
"They don't like it much when immigration, security and public order are being tackled," Cautres said.
But the spectre of the Gilets Jaunes crisis may, in the end, help him.
Cautres said: "The idea for his MPs may be to form a bloc on these issues. They all know that the situation is highly flammable so I don't think they're particularly keen to play with matches."