The emergence of strong centrist candidates in France's presidential race, led by three-time contender Francois Bayrou, could make life harder for President Nicolas Sarkozy as he fights to rally the right around his re-election bid.

With the election five months away, the race seemed to have solidified into a straightforward battle between Socialist front-runner Francois Hollande, a weak but reviving Sarkozy and far-right chief Marine Le Pen in the role of kingmaker.

That premise mostly holds true. But the entry of Bayrou and former prime minister Dominique de Villepin in recent days has lengthened the list of centrist contenders and complicated the picture.

The pair have started to preach an attractive middle ground between growth and austerity, taking up precious media air time and sniping at the mainstream candidates.

The chances that one of them will break through to the election's final round are slim, but the chances that they will score well and divert votes to Hollande in the run-off are too strong to ignore, pollsters say.

Despite the fact that many of these voters have right-leaning instincts, they are more likely to vote for Hollande than Sarkozy when it comes to the final round, said Gael Slimane, an analyst at pollster LH2.

There is disappointment with Sarkozy on the right... That can make problems for his stock of votes in the second round.

Sarkozy has enjoyed a slight rebound in the polls in recent weeks, buoyed by positive views of his leadership on Libya and in the euro zone debt crisis. But the recovery is stalling amid worries about the safety of France's AAA credit rating.

Most pollsters see Hollande beating Sarkozy in the final round of the election next May, even though he has lately lost some ground to the incumbent.

The centrists' message varies on the details. But all speak with one voice to condemn Sarkozy's debt and deficit legacy, his efforts to rally support on the far right and his handling of the euro zone crisis.

DAMAGE POTENTIAL

Villepin, a longtime Sarkozy foe who announced his candidacy on Sunday, is not explicitly in the centrist camp but appears slightly to the left of Sarkozy. In Sunday's LH2 poll his support registered at a low of around 1 percent.

Pollsters say there is room for improvement - at Sarkozy's expense. He's a candidate who can do a lot of damage, said Jean-Daniel Levy, analyst at pollster Harris Interactive. A few months ago he was polling 6-8 percent, which shows that he has a lot of potential appeal, especially on the centre-right.

A group of other candidates, which includes Sarkozy's former defence minister Herve Morin and his former housing minister Christine Boutin, have made minor poll gains. Polls show their supporters also defaulting to Hollande.

Far more threatening is Bayrou, a former history teacher and son of farmers from southwest France who has carved a wide niche for himself between mainstream currents as a pro-European, fiscally disciplined centrist candidate.

During a 2007 presidential bid, his second, Bayrou drew an unexpected 18.7 percent of the popular vote as the only candidate to run on a platform of reducing France's deficit.

With many analysts suggesting that France could be days away from losing its precious AAA credit rating, voters are likely to reward Bayrou's prescience on his third run, said Levy.

He was already talking about this in the last election, so today his position does not look like a construction. That consistency is an asset for him, he said.

The candidate that the right-leaning Le Point magazine recently dubbed l'Emmerdeur -- or the Pain in the Ass -- on its cover has enjoyed a strong run in the polls, gaining 6 percentage points since November to reach 13 percent of the vote, according to LH2.

In 2007, Bayrou's supporters split evenly between Socialist candidate Segolene Royal and Sarkozy in the final round. This time polls suggest his backers will rally much more forcefully behind Hollande, because Bayrou has made a point of blaming Sarkozy for France's fiscal woes.

The Bayrou vote is more than ever a vote against Sarkozy, BVA's Slimane said. Bayrou has built his campaign on anti-Sarkozy feelings, and many of the people following him are deeply anti-Sarkozy. It won't be a stretch for them to vote for Hollande.

(Editing by Mark Trevelyan)