French politics will stray into uncharted, transatlantic territory this weekend when left-wing voters will be invited to take part in the country's first "open" presidential, primary election.
If pollsters are correct, little doubt remains about the identity of the centre-left challenger to President Nicolas Sarkozy next spring. In the last few days, the former Socialist party leader, François Hollande, 57, has extended his lead in the opinion polls.
Sunday's vote will be the first time that any French political party has opened its choice of presidential candidate to all comers. Anyone who has a national voting card can cast a ballot, so long as they pay a minimum of €1 to the Parti Socialiste and declare themselves to share the "values of the left".
Socialist party officials hope that as many as two million people will take part. Since the breadth and political chemistry of the electorate is uncertain, opinion polling is more hazardous than usual. The five candidates who are trailing behind Mr Hollande, especially the party leader, Martine Aubry, predict that the pollsters will be badly embarrassed this weekend. Some of Mr Hollande's supporters say that he might reach 50 per cent of the vote on Sunday, enough to be elected outright and pre-empt the two-candidate second round seven days later.
One poll this week showed Mr Hollande with more than 50 per cent support among Socialist party members and 46 per cent support among those left-wing sympathisers who say that they are certain to vote.
Despite the American origins of the primary idea, this will be a very French kind of election. There has been little controversy and a series of worthy but dull television debates. The intention is to create nationwide, rather than partisan, momentum behind a centre-left challenger long before the presidential election next April and May. Since both Mr Hollande and Ms Aubry are predicted to defeat Mr Sarkozy easily in the spring, the experiment can be said, so far, to have been a success.
Mr Hollande has been the great beneficiary of the aborted ambition of the former Socialist Finance Minister, Dominique Strauss-Kahn. DSK was the overwhelming favourite to win the Socialist primary when he was arrested in New York in May on attempted rape charges, which have since been dropped.
Mr Hollande, humorous, likeable, sharp-minded but unexciting, has also benefited from the debt crisis, the unpopularity of Mr Sarkozy and the corruption investigations which have swirled around the President's friends. The former Socialist leader promises to be a "normal, honest and moral" President. Above all, he offers himself as a moderate, prudent man who can be trusted to rebuild the nation's debt-laden finances.
Only in the last few days has the primary campaign burst into life. The magazine L'Express alleged that the internal security services had been ordered to investigate the private life of Mr Hollande's girlfriend, Valérie Trierweiler, 54, a journalist with Paris Match and a political interviewer. She has been Mr Hollande's partner since he broke up with the former presidential candidate, Ségolène Royal, in 2007.
Runners and riders
Jean-Michel Baylet According to latest opinions polls, support for the leader of the Radical Left Party is almost non-existent.
Martine Aubry The Socialist Party leader and daughter of former European Commission President Jacques Delors is seen as the second strongest candidate behind Mr Hollande. She rose to prominence as labour minister between 1997-2000, but critics have called her uninspiring.
Manuel Valls The Spanish-born mayor of Evry, one of the poorest towns in France, is seen as a favourite among the party right. He has been compared to Tony Blair during the launch of New Labour.
François Hollande Opinion polls rate the Socialist Party's former leader as favourite to run against President Sarkozy in 2012. His supporters say he is a strong campaigner but critics say that he is unexciting choice.
Arnaud Montebourg Segolene Royal's spokesman during the 2007 presidential election is running as a champion of French industry and critic of globalisation.
Ségolène Royal Fighting back has become a career trait for Ms Royal. She has bounced back from her failed 2007 presidential bid and the split from her former partner Mr Hollande to run as third favourite in latest opinion polls.