As they rally their supporters for Sunday’s first round of the French presidential election, Nicolas Sarkozy and François Hollande already have one eye on the decisive run-off, set for May 6.

For now, the priority for the two frontrunners is to come out top in round one – and to ensure they are not upended by a surprise surge by one of the other eight candidates, as happened in 2002 when Jean-Marie Le Pen, then leader of the far-right National Front, stunned the country by knocking out Lionel Jospin, the Socialist candidate.

But assuming, as all the polls suggest, that the centre-right president and his Socialist challenger make it through, they will both face a delicate task in the second round as they seek to build a majority from voters who on Sunday will have backed parties ranging from the extreme right through the centre to the extreme left.

To a large extent, round two is a different election. As an oft-quoted saying has it: “In the first round people vote with their heart, but in the second they vote with their head.”

On the face of it, Mr Sarkozy has the bigger problem. With the polls universally showing him well behind Mr Hollande in a run-off, he is anxious to win the first round to gain some much-needed momentum.

“He needs to be four or five points ahead of Hollande in the first round to give himself a chance,” said one anxious sympathiser.

This week, Mr Sarkozy has appealed directly to supporters of the National Front to vote for him. “A vote for Marine Le Pen will only help Mr Hollande,” he told a rally on Wednesday evening.

He needs to woo NF voters for two reasons: first, to ensure Ms Le Pen does not do to him what her father did to Mr Jospin; and secondly to get as many of those who stick with her on Sunday to switch to him in round two as he can.

At the same time, Mr Sarkozy’s team has moved to attract supporters of François Bayrou, leader of the centrist MoDem party, suggesting that Mr Bayrou could be prime minister in a new Sarkozy government.

It is an awkward balancing act, for Bayrou supporters tend to be repelled by the hard-right positions Mr Sarkozy has adopted to attract NF voters. But the centre is where Mr Sarkozy senses he could make serious inroads on Mr Hollande before the May 6 vote.

The most striking feature of the campaign so far has been the rise of Jean-Luc Mélenchon, leader of the Left Front, a group that includes a resurgent Communist party. Mr Mélenchon has made clear he will vote for Mr Hollande in round two – as will the vast majority of his supporters.

But the rise of the radical left and the potential influence it could have on Mr Hollande’s policies is a big concern to centrist voters worried about the country’s fragile public finances and flagging economy.

Mr Hollande insists there will be no negotiation on his policy programme, which commits him to balancing the budget. But in a sign of the pressure from the radical left, on Thursday Mr Hollande said he was willing to give “a little boost” to the minimum wage above the statutory indexation to which it has been limited under Mr Sarkozy – an issue on which Mr Hollande had been careful not to make promises. A big rise in the salary floor has been a key part of Mr Mélenchon’s campaign.

Despite the weight of polls against him, Mr Sarkozy believes he can persuade a “silent majority” of French voters (code for undecideds and supporters of Mr Bayrou) that a vote for Mr Hollande will put the country at risk. “Everyone in France is aware of what is happening to Spain,” he said on Thursday. “It is the result of seven years of socialism with the same policies Mr Hollande is proposing.”

The president is anxious to move on from the “cacophony” of the first round to a head-to-head contest. Then, his aides believe, he can expose what they call Mr Hollande’s evasiveness on policy. He particularly relishes the planned television debate between the two finalists – there has been no direct confrontation between candidates in the first round.

But first, both men have to get through Sunday’s vote.

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