The candidate must deal with the legacy of President Francois Hollande, whose ratings plummeted with his failure to turn the economy around. It appears a near-certainty that no Left representative will get beyond the April 23 first round.

The focus of campaigning -- not only for the Socialists but also the election front-runners -- is likely to focus on France's moderate left, a vast electorate spanning factory and office workers, shopkeepers and Paris professional elites.

Voters will whittle down a field of seven in a left-wing primary organised by the ruling Socialist majority on Sunday. The winner and runner-up will go into a January 29 runoff to decide who will be the Left's standard-bearer in the election.

Conservative Francois Fillon and far-right leader Marine Le Pen are seen likely to meet in the knockout on May 7 - and both have an interest in the outcome of the Left primary.

Independent centrist Emmanuel Macron, 39, an ex-economy minister who is drawing huge crowds with a programme offering an alternative to the classic Left-Right divide, is rising fast, opinion polls show.

But it will be harder for Macron to corral the centre vote if Socialist ex-prime minister Manuel Valls, who moved policy sharply to the right under Hollande, wins the nomination, as an opinion poll on Thursday suggests he will.

CENTRE GROUND

Valls, 54, implemented Hollande's pro-business policies and incurred the anger of many in the Socialist party for his reforms. He has polled far higher nationally than other left-wing candidates.

An authoritarian interior minister before Hollande named him prime minister, his appeal to the neutral and centre ground could be competition for Macron and hurt the ex-banker's prospects of getting to the May 7 knockout.

"There are political forces at play which don't want these primaries to be held in good conditions," Valls said about Macron. "Whoever wins will have to unite, but not with old-fashioned political deals," he said.

If the choice falls on a candidate further to the left, such as Arnaud Montebourg or Benoit Hamon, this will help Macron.

Both men, who were thrown out of government for criticising Hollande's economics, have performed well in televised debates.

Hamon was found the winner of the final TV debate before the vote by a flash poll immediately afterwards.

The Socialists trail behind Fillon, Le Pen, Macron and Jean-Luc Melenchon, a veteran campaigner of the "hard" left, for the presidential vote, according to a new opinion poll published by Le Monde on Thursday.

The political air is thick with talk of possible alliances involving a Socialist candidate and Macron. He refused to take part in the primary even though he served in Hollande's government until last year.

"If Manuel Valls wins, there will be scarcely any daylight between him and Macron and that will give Melenchon greater political space," Socialist party chief Jean-Christophe Cambadelis told Reuters.

"If it is Hamon or Montebourg, the difference with Melenchon will practically disappear and that will give greater room for manoeuvre to Emmanuel Macron," he said.