Francois Hollande, the unexpected favourite, or Martine Aubry, the experienced leader. France's Socialists battle it out Thursday in a televised debate ahead of their US-style primary to pick a candidate for the 2012 presidential election.
The two frontunners will appear alongside four outsiders to explain their projects and their differences and try and persuade voters to pick them in next month's primary -- the first of its kind in France.
The primary will not be limited to members of the Socialist Party (PS), as was the case ahead of the last presidential vote in 2007.
Any French citizen who declares that he or she is committed to left-wing ideals and hands over a euro can take part.
The electorate of the primary is therefore unknown, which makes trying to predict the result very difficult, even if Hollande, 57, has been ahead in the polls since Dominique Strauss-Kahn was put out of the running.
Hollande was given a clear lead in a poll published Tuesday in the left-wing daily Liberation. It said that 40 percent of left-wing sympathisers would vote for him, against just 22 percent plumping for Aubry.
Hollande is a member of parliament for the rural Correze department of central France and was head of the party for 11 years.
He has said he wants a calm debate on Thursday evening by the candidates, who come from a party that is known for its bitter in-fighting.
"These debates should not push us to confrontation. There is no major contradiction between us. We are in the same party, we voted for the same project and we will be together in 2012," he told freesheet newspaper Metro. The Socialists are hoping that hundreds of thousands of people will head to the voting centres specially set up for the primary vote held in two rounds on October 9 and 16.
Party officials have said they expect around a million people to take part.
Such a large turnout would create momentum for the winning candidate and would give him or her an advantage over Nicolas Sarkozy.
The right-wing president has not officially declared that he will run for a second mandate in the vote to be held next April and May, but few doubt it.
"You can rely on me to make it (Thursday's debate) lively," said Manuel Valls, one of the outsider candidates who is on the right of the Socialist primary.
"I will be respectful of my friends and competitors, but I want to know each one's position on how to reform pensions, on public spending, on tax," he told 20 Minutes newspaper.
With Europe's sovereign debt crisis rattling markets and threatening the future of the euro, the main Socialist candidates have rallied to the necessity of rigorous control of the public purse.
Yet the party has committed itself to bring the retiremement age back to 60, from the 62 Sarkozy raised it to, and it also says it wants to create state-subsidised jobs for young people.
In third place in the polls is Segolene Royal, who was the Socialists' candidate last time round in 2007.
The feisty and unpredictable Royal last week put an abrupt end to the primary candidates' tacit non-aggression pact.
"Francois Hollande's weak point is his lack of action," she said. "Can the French point to a single thing he had done in thirty years of politics?" she asked, pointing to Hollande's lack of ministerial experience.
The jibe was all the more pointed as she was Hollande's partner until 2007. The couple had four children together.
Aubry is hoping that the TV debate will dynamise her campaign for the primary, which so far has struggled to find a rhythm and has raised doubts even within her own entourage.
The resolutely left-wing Arnaud Montebourg was expected to try and carve out a place for himself during the debate with his arguments against globalisation.
Jean-Michel Baylet, the leader of the small Socialist-allied Radical Party of the Left, is the sixth candidate in the debate.