As François Hollande delivered a rousing speech in a Paris theatre on why he should be the next president of France, the great and good of leftwing politics and culture cheered.

Photographers focused on Benjamin Biolay, the French singer who recently shot down rumours he had had an affair with the first lady, Carla Bruni. But necks were craning to spot another, new household name: where was Valérie?

Hollande, 57, the rural MP and former Socialist party leader, is favourite to win next month's primary race to choose a challenger to Nicolas Sarkozy for the 2012 presidential election, ahead of his ex-partner Ségolène Royal. The bespectacled, portly joker and determined tax-reformer has gone on a diet and styled himself as "an ordinary guy" in an attempt to counter fears that he is too dull to lead France.

And yet his incredible Mills & Boon love life is still overshadowing the Socialist race. His new partner, Valérie Trierweiler, a political journalist who once covered the Socialists for Paris Match magazine, has been catapulted into the limelight in the latest chapter of an extraordinary saga of sex, lies and opinion polls.

Hollande and Royal were once the power couple of the French left. He led the Socialist party for 11 years. She was a minister, then head of the western region of Poitou-Charentes. They never married, considering it too "bourgeois", but they often posed around the breakfast table with their four children.

Then everything imploded at the last presidential election in 2007. Hollande had met someone else and Royal ran as the Socialists' first female presidential candidate, trampling his ambitions.

The couple's secret break-up and personal rivalry was blamed for losing the election to Sarkozy. "It was their relationship far more than Sarkozy that broke down the boundaries between private and political life and continues to do so," said one political journalist.

After the defeat, the pair announced their split. But it emerged that they had in fact separated years before but had hidden it from the country. Hollande was gutted that Royal had outstripped him in the polls and run for president. Royal's supporters were exasperated by what they saw as Hollande's fatal lack of support; her spokesman even announced on TV that "her only problem is her partner". It also emerged that Hollande had fallen in love with Trierweiler, and set about starting a new life with her. The bitter and acrimonious disintegration of the relationship became a metaphor for the Socialist party itself falling apart.

Now a new round of rivalry between Hollande and Royal is playing out as both run in October's primary race to choose a Socialist candidate. This time, Hollande is the favourite, followed by Martine Aubry, mayor of Lille and most recent party leader. Royal is polling in third position but her support could be decisive if she is eliminated and throws her weight behind either Hollande or Aubry in a second round run-off.

The French media is obsessed with how far Hollande and Royal might go in publicly attacking each other in the first live primary TV debate on Thursday. Hollande is keen to avoid any confrontation. But Royal last week told Le Figaro: "Can French people actually name anything he's achieved in 30 years of political life?"

Serge Raffy, author of a much-talked-about Hollande biography released this week, said: "The Hollande-Royal relationship was a kind of political romance saga that had never been seen before anywhere in the world. Here was a political couple who for two years, from 2004 to 2006, competed for the highest echelons of power, to become French president. It's unique. Even the Clintons never went for power at the same time.

"Their relationship paid the price. They couldn't bear the rivalry and they separated. It was very difficult for him, he suffered [when she ran for president].

"He had ambitions but he wasn't in a position to run, she overtook him. They mixed private and public life like we'd never seen before in France. They avoided the paparazzi and kept their separation secret for two years."

Raffy thinks they are now on good terms. "The rancour has gone. It's simply two political animals facing each other. But they know each other by heart."

Some believe they have made a pact that if one of them wins the other will offer support. Certainly a photograph of them at a rally in May appeared to show Hollande making eyes at Royal and her blushing and smiling.

The picture reportedly irked Trierweiler, who complained to the editor of the paper that published it. This in turn raised eyebrows in the press pack about Trierweiler's role in the campaign. She currently hosts a political TV show.

During Hollande's speech at the Socialist party conference in La Rochelle last month, Trierweiler appeared in the press enclosure and, one correspondent whispered, was wearing a press badge.

Some think she must choose her role – neutral political journalist or future first lady. She and Hollande appear in public together and he has said "she is the woman of my life", which in turn was said to have upset Royal.

Raffy's biography revealed what a small world journalism and politics is in France: in 1992 when Royal gave a controversial interview from a maternity ward just after giving birth to her youngest daughter, it was Trierweiler who conducted it. No one knew at that stage what the future would hold.

Hollande and Royal don't want the scrutiny of their every gesture towards each other to overshadow the political debate on education, tax and how to beat Sarkozy. But the couple remains the source of endless election gags. When wince-inducingly naff soft-rock was played before Hollande came on stage in Paris, one political correspondent tweeted: "Did Ségolène keep the Tina Turner albums?"

Finding a challenger to Sarkozy



The Socialist primary race to choose a presidential candidate is the first exercise of its kind in France. Anyone on the electoral register can vote if they pay €1 and sign that they adhere to the ideals of the French left. The first round is on 9 October, followed by a second-round runoff on 16 October.

Before Dominique Strauss-Kahn was arrested in May on attempted rape charges in New York, he was considered to be an almost certain winner. Now the race is wide open.

Thursday sees the first of three TV debates between the six candidates. François Hollande, MP for Corrèze and party leader until 2008, is broadly centre-left and promises to defend French youth, fairer taxes and cut the French deficit. Polls suggest he is best placed to beat Sarkozy.

Second is Martine Aubry, 60, mayor of Lille and the most recent party leader, who as an architect of the 35-hour week is further left and has promised a more "caring" society with strong public sector and an increase in arts spending.

Ségolène Royal, the head of the western Poitou-Charentes region who lost to Sarkozy, left, in 2007, claims she still has a strong following across France with her views on open democracy and public consultation.

Manuel Valls, an MP and mayor in the Paris suburbs has taken a hardline view on security and spending cuts and is seen as towards the right of the party; Arnaud Montebourg, an MP in eastern France has taken the most leftist stance, calling for "deglobalisation" and an end to bank speculation.

Jean-Michel Baylet, leader of the small centre-left Radical Party of the Left, is the only non-Socialist running.