French politics will stray into uncharted 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 year. This week, the former Socialist party leader, Francois Hollande, 57, extended his lead in the opinion polls.
Tomorrow'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 ($1.70) to the Parti Socialiste and declare themselves to share the "values of the left".
Socialist party officials hope that as many as 2 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 Hollande, especially the party leader, Martine Aubry, predict that the pollsters will be badly embarrassed this weekend. Some of Hollande's supporters say that he might reach 50 per cent, enough to be elected outright and pre-empt the two-candidate second round a week later.
One poll this week showed 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.