Four days to go to the first round of France’s presidential election, and it’s looking closer than ever.

The top two candidates on Sunday will fight it out in the second round two weeks later. Most probably they will be the centre-right UMP’s Nicolas Sarkozy and the Socialist Party’s Segolene Royal. Opinion polls have consistently shown Sarkozy in the lead, but Royal has been gaining in the last week.

Yesterday’s Ipsos daily survey has Sarkozy leading Royal 28.5% to 25% in the first round, and 52%-48% in the second round. The CSA poll in Le Parisien says 27% to 25% in the first round, and level at 50-50 in the runoff.

Royal’s growing support is bad news for centrist-liberal candidate Francois Bayrou, who would be favoured to win a runoff against either Sarkozy or Royal, but has to get ahead of one of them in the first round. His support looks to be stuck just under 20%.

Sarkozy seems to have been shoring up his base on the right, sounding more like a traditional conservative in recent weeks and bidding for the supporters of far-right candidate Jean-Marie Le Pen. That in turn has led to increased anxiety on the centre and left, and a move to unite to stop Sarkozy.

Hence former Socialist prime minister Michel Rocard proposed at the weekend an alliance between Royal and Bayrou, whereby each would commit beforehand to support whichever of them won through to the second round.

Both turned down the idea: Royal because she doesn’t want to flag the possibility of coming third, and Bayrou because he has nothing to gain from it. If he does make the second round against Sarkozy, he knows the Socialists will support him anyway.

But if, as expected, Bayrou runs third on Sunday, then he will be the kingmaker. The race between Royal and Sarkozy is so close that a clear endorsement for one of them would probably swing it.

And if Bayrou, whose party is traditionally seen as part of the right, can bring himself to back Royal, it will herald a radical new era in French politics.