French candidate for the 2011 Socialist party primary elections Francois Hollande, right, waits to vote vote in the second round of the party's primary election to chose a candidate for the 2012 French presidential elections,in Tulle, central France, Sunday, Oct.16, 2011. (AP Photo)
Francois Hollande or Martine Aubry; Whichever of the pair wins the Socialist Party nomination will immediately become France's frontrunner in the race, as polls show the centre-right incumbent trailing either in April and May's two-round contest.
The vote is also France's first US-style open primary -- any elector who says he or she supports the ideals of the left can vote -- and a big turnout could serve as a springboard for the campaign proper.
Last Sunday, a bigger than expected 2.66 million voters turned out for the first round, which was won by 57-year-old lawmaker and former party leader Hollande, with only a narrow nine-point leader over Aubry.
Hollande has since won the backing of the four defeated candidates, and enters the second round as favourite, but Aubry mounted a tough fightback, branding him a soft centrist without the steel to defeat Sarkozy.
Aubry, 62,the former labour minister who gave France is 35-hour working week and mayor of the city of Lille, has also attacked Hollande's lack of executive experience -- he has never served at cabinet level.
Hollande has tried to turn the attacks to his advantage, accusing Aubry of undermining party unity and suggesting that his lack of a track records makes it easier for him to run as a candidate of change.
Sarkozy's camp was wrong-footed by the primary. Some of his supporters grudgingly admit it served as a good shop window for the Socialists, but the president himself dismissed it as alien to French political tradition.
Leaders of the ruling UMP have mocked the policies on display in the left's primary, portraying them as a throw-back to the 1980s, but all four televised debates between the candidates drew large audiences.
The suprise of the first round was the break-through of Arnaud Montebourg, who came in third with 17 percent of votes on a ticket of reinforcing the regulation of banks and markets and of trade protectionism.
The right had hoped that the primary format would sow discord between the six hopefuls, triggering the infighting for which the Socialist Party is infamous and hobbling the victor before the general election.
In fact, the first round remained relatively civil, despite a history of personal animosity between the frontrunners.
Aubry succeeded Hollande as Socialist Party general secretary and has since let it be known that she found the organisation in a sorry state.
Hollande was the partner of fourth-placed challenger and defeated 2007 Socialist candidate Segolene Royal for 30 years, raising four children, but split from her before the last race and moved in with his girlfriend.
Meanwhile, the whole party was left shell-shocked in May when the former favourite for the nomination, IMF chief Dominique Strauss-Kahn, was accused of attempted rape by a New York hotel maid and a young French writer.
Prosecutors have since dropped both investigations, but Strauss-Kahn left the race, pausing only to undermine Aubry by declaring they had had a pact that she not run if he did, effectively tagging her as a substitute candidate.
Despite the bad blood, the campaign only truly turned bitter in the closing straight, when Aubry attempted to close down Hollande's narrow but consistent lead by tacking to the left and branding him "soft".
Historically, both she and Hollande come from the party centre ground.
In 1995, when the last Socialist president Francois Mitterrand left office, they were both apostles of modernising former European Commission chairman Jacques Delors -- Aubry is his daughter, Hollande his protege.
In all, 9,474 voting stations across France opened at 7:00 am (0500 GMT), to close at 5:00 pm, with partial results expected less than two hours later.
Short link: