Any French citizen who pays one euro and declares a commitment to left-wing ideals will be able to vote in next month's primaries and tonight the potential candidates will outline their positions in a carefully stage-managed exercise.
It was widely assumed that former IMF boss Dominique Strauss-Kahn would be chosen as Socialist Party candidate for the May 2012 Presidential elections, until his dramatic arrest in New York on charges of attempted rape which were later dropped.
Martine Aubry, daughter of former European Commission president Jacques Delors, threw her hat into the ring, many say reluctantly, when it became obvious that Strauss-Kahn would not stand.
She suffers from being seen as a "substitute", as well as being a dull television performer, but her low-key approach could prove popular.
Ahead in the polls is former party leader François Hollande, who was the first formally to announce his candidacy for the primaries, and has tried to cultivate an image of "The Common Man", with strong links to his rural constituency.
Television viewers will be interested in the chemistry between Hollande and one of the other hopefuls, Ségolène Royal, the unsuccessful Socialist candidate who lost to right-wing Nicolas Sarkozy in the last presidential elections.
Royal and Hollande were partners until 2007 and have four children together.
Her chances are hard to predict. She does not sit easily either on the right or the left-wing of her party, but is much better known to the general public than her competitors, and trailing in the polls, she could feel she has all to play for in tonight's debate.
Last week, she extravagantly flouted the candidates' non-aggression pact, saying of her former partner "Can the French point to a single thing he has done in thirty years of politics?".
Tonight, he could choose to hit back.
Three other Socialist politicians bidding for the candidacy are thought to have little chance.
Manuel Valls is on the party's right wing, Arnaud Montebourg is on the left, and Jean-Michel Baylet is leader of the Socialist-allied Radical Party.