The French Socialist party will test its reputation for inflated egos and in-fighting on Tuesday when it opens its primary race to chose a presidential candidate to challenge Nicolas Sarkozy in 2012.

Recent polls have shown the Socialists would trounce the unpopular Sarkozy if an election was held now. But the French left – which has not won a presidential vote since François Mitterrand in 1988 – is painfully aware of how poll leads can evaporate at the ballot box. Its choice of candidate is crucial if it is to run a personalised campaign attacking Sarkozy as unprincipled, ineffective and power-obsessed.

The left is still reeling from the forced exit from political life of its one-time favourite, Dominique Strauss-Kahn, after he was charged with the attempted rape of a New York hotel maid last month. If the party is to win the moral fight against Sarkozy, it must now have a candidate whose track record and behaviour is irreproachable. European politics in general have shifted firmly to the right. To buck this trend and get elected, the party needs to field a broad-church candidate whose appeal reaches centrists and the right.

The selection contest, which begins on Tuesday and ends with a vote in October, boils down to two key characters. François Hollande, 56, the current favourite, is a former Socialist party leader and MP in rural central France who drives a moped and styles himself as an "ordinary guy".

Hollande was once seen as a portly, jovial joker before going on a crash diet and radically smartening up his look after splitting from his long-term partner, former presidential runner Ségolène Royal. His policy ideas are centre-left. His supporters argue that he is down to earth and the polar opposite of Sarkozy. His detractors say he has never held a ministerial post and lacks international experience.

Hollande declared his fierce ambition to be president last March and has criss-crossed France in old-fashioned campaign meetings that won over the media and saw him ranked the most "presidential" politician in France in the latest poll for Le Monde.

The party's current leader, Martine Aubry, is expected to enter the race to challenge him. The 60-year-old mayor of Lille and staunchly leftwing architect of France's 35-hour week, will run a campaign to be France's first female president. One ally told the French newspaper Liberation that she would style herself as a kind of "Angela Merkel of the left": stern, serious and slightly authoritarian. Her team thinks that in a "macho country like France", unless she pushed herself as hard and domineering, she would be dismissed as incapable of running the country.

Aubry's first task is to convince France of her "presidential standing" and that she is genuinely ambitious. Before the Strauss-Kahn sex attack case, she had been happy to let him run instead of her and has seemed reticent about being pushed into the front line.

"We know that she's totally opposed to Hollande being the Socialist candidate, but does she actually want to be president herself?" asked one diplomat.

The third main candidate is Royal, still running despite her defeat by Sarkozy in 2007.

Sarkozy has seen a modest rise in his approval ratings in recent weeks, due to a slight improvement of France's economic growth forecast and minimal drop in unemployment. speech on Monday to showcase his ideas for kick-starting the economy. But he remains widely disliked, and international efforts such as the Libya intervention have not had a significant impact on his ratings at home.