I happened to be in Paris when the idea of a "primary of all the left" was launched, so I know that it created quite a buzz--but I never quite believed in it. Now Daniel Cohn-Bendit confirms my understanding of why it would never work: Quite simply, the various components of the Left hate each other more than they want to win an election, especially if there is the slightest possibility of being obliged to coalesce around François Hollande, who has become the bête noire of everyone except his own inner circle. As long as the price of entering a left primary is a pledge to support the eventual winner, even if it is Hollande, it's a non-starter. Meanwhile, the remaining Hollandais refuse to commit their man to participate in the primary if he decides to run.
The only way out would be an early withdrawal by Hollande, which doesn't seem to be under consideration, even though it's hard to see how he thinks he can possibly win. Cohn-Bendit considers a deux ex machina in the form of Nicolas Hulot, but Hulot's is not a candidacy I can believe in. So at this point the Left looks certain to be headed for utter fragmentation in the first round, leading to a total rout, with the Socialist Party reduced to historic post-Epinay lows. This will mark the definitive liquidation of the Mitterrand era and a complete recomposition of the French party system.