Heavyweight centrist François Bayrou, the eternal "third man" of French politics, has surprised supporters by saying he would not stand for president but instead offer an alliance with Emmanuel Macron.
Mr Bayrou said France was at "extreme risk" and needed what he described as an "exceptional response".
After weeks of suspense, the 65-year-old president of the Democratic Movement (MoDem) party and a veteran of three previous leadership elections, had been expected to announce he would join the presidential race.
Instead, he said he would not stand but offered to join forces with Mr Macron (39), the former Socialist economy minister who is standing on a centrist ticket.
The announcement on Wednesday, described as an unprecedented move, took French political pundits and rival candidates by surprise. Many see it as a major potential boost for Mr Macron, whose campaign had appeared to stall.
Polls suggest the bulk, though not all, of Mr Bayrou’s support – thought to be worth 5-6 per cent of the vote in a race that may come down to two or three percentage points – will transfer to Mr Macron, increasing his chances of advancing to the second round run-off ahead of his scandal-hit conservative rival, François Fillon.
“I have two paths, to stand myself or to look for an unusual solution. I have decided to offer Emmanuel Macron an alliance,” Mr Bayrou told a press conference.
“Perhaps it’s a sacrifice for me, but I feel there are times one has to rise to the seriousness of the situation and consider how to get out of it. It’s not a time for me to think of myself, but of my country.”
Faced with the far-right Front National candidate Marine Le Pen – currently leading polls for the first round vote – whom he described as the "threat and major danger for our country and Europe", and Mr Fillon, who has been hit by allegations over jobs given to his wife and children , Mr Bayrou said the French were "disorientated and despairing".
“Never in the 50 years past has the democracy in France known such a situation,” Mr Bayrou said, adding that French politics was riddled with “practices that would not be expected anywhere else”. The presidential campaign rocked by scandals had left him “stupefied” and “made a mockery of France”, he added.
“To the right, affairs have been uncovered that reveal not just the existence of privileges and tendencies but the tacit and almost unanimous acceptance of them.
“For a long time it’s been repeated that ‘everyone does it’. But I can stand here and say it is not true and it is defamatory for the vast majority of elected representatives.”
Mr Bayrou said one of his conditions for an alliance with Mr Macron, whom he described as “brilliant”, would be a major clean-up of France’s political life.
“French people feel politicians’ words count for nothing. They have no confidence in the words and promises they hear ... we have to convince the French our actions can match our words. It’s a good time to do it even if it is a sacrifice,” he said.
Mr Bayrou, who was an education minister in a centre-right government in the 1990s, said he had spoken to Mr Macron a week ago and insisted it should be an alliance and not a subjugation of the “French centrist movement”.
“Perhaps this can be the foundation of a new approach in French politics,” he added.