The field of mould-breaking, job-creating, tree-hugging, consensus-seeking, debt-cutting, Europe-doubting politicians has grown more crowded in France's presidential elections. Now there are three.

For several weeks, Nicolas Sarkozy, the boisterous presidential candidate from the ruling UMP party, and Ségolène Royal, the elegant Socialist party challenger, thought they were the only electoral party in town. But François Bayrou, the rustic leader of the centrist UDF party as famous for his tractor as his political ideology, has rudely gatecrashed their bash.

Mr Bayrou's emergence as a serious contender has added a fascinating new dimension to the political campaign and could have a significance far beyond these presidential elections. There is already excitable chatter in Paris about a political realignment of the left that could be just as important as who next occupies the Elysée palace.

At the Zenith stadium on Wednesday night, Mr Bayrou was cheered to the heavens by 7,000 orange-clad supporters as he called for a "peaceful revolution", an overhaul of the country's political institutions, and the foundation of a Sixth Republic. Denouncing the two mainstream parties for monopolising government for a quarter of a century, he promised to give power back to the people. In their giddier moments,

Mr Bayrou's supporters have even suggested France is on the brink of its own Orange Revolution, comparable to the upsurge of people power in Ukraine in 2004.

Mr Bayrou's popularity has been inflated by the fact that he is neither Ms Royal nor Mr Sarkozy, who have both unnerved voters in different ways. Mr Bayrou has emerged as an acceptable protest candidate, capitalising on the anti-establishment anger tapped by Jean-Marie Le Pen, the National Front leader, in the 2002 presidential elections. But this has not stopped Mr Bayrou from stealing the best lines of his opponents; indeed, it is almost as if he has copied and pasted parts of their speeches into his own. Like Ms Royal, Mr Bayrou has stressed the need for a new style of participative democracy. Like Mr Sarkozy, the traditionally pro-European Mr Bayrou has sounded a euro-quizzical note, raging against unfair foreign competition and the need for the European Union to be "protective, if not protectionist".

The UDF leader's political gyrations have astonished those who have followed his career. Previously, he had positioned his party to the right of centre and for most of the current parliamentary term he had supported the UMP-led government. But sensing a gap in the political spectrum, he has smartly reinvented himself as a social democrat. His repositioning appears shamelessly opportunistic. Yet Mr Bayrou might just do to the Socialist party what it has not dared do itself: drag it into the 21st century.

There are already rumblings in Socialist circles that a realignment of politics is desirable. This week a group of leading party sympathisers published a "Manifesto of the Gracchi" calling for the Socialist party to ally itself with the UDF to create a forward-looking, pro-European social democratic movement. The unsigned manifesto, published in Le Point magazine, said Socialist party leaders had abandoned their supporters by failing to adapt to the times.

"In private they know exactly what they have to do: follow the reforms successfully pursued by social democrats in most other European countries. But in public they insist on publishing a catalogue of promises from another century," the manifesto read.

"Under the pressure of its left wing, Rue de Solferino [party headquarters] claimed that in France it was different: for the Socialist party to win it must reject the market (even if regulated), globalisation (even if domesticated) and modernisation of the state (even if negotiated). Thanks to Francois Bayrou for brilliantly demonstrating the contrary!"

Such a political realignment would certainly be logical - and could prove a formidable electoral bloc to counter Mr Sarkozy. But that does not mean it is going to happen.

Pascal Perrineau, director of Cevipof, a political studies institute, says the manifesto's signatories appear to be "confusing desire with reality". The Socialist party was refounded in the 1960s on an explicit rejection of centrism and this thinking has seeped into its members' bones, he says. "The history of the Socialist party shows that this discourse [of realignment] is not impossible but is very difficult, especially in a very short time."

Yet Ms Royal is herself a strange Socialist, a woman of the left with the values of the right, who has been attracted to Scandinavian-style social democracy. She has been highly critical of her own party. In effect, she only won her party's nomination last year by campaigning against the party apparatus and drawing new members to her cause. She may yet be tempted by an electoral alliance if she were convinced that she could not beat Mr Sarkozy on her own.

But by the end of this week there was a distinct sound of hissing emerging from the Bayrou bubble and Ms Royal was the one standing nearby with a pin. In spite of the battering she has received in the media, she is a resilient campaigner. This absorbing election campaign has thrown up many surprises. It would be no surprise if there were plenty more before the first round of voting in a month's time.

The writer is editor of the FT's European edition