Over at the TPA blog my colleague Lee Rotherham points out that the Lisbon Treaty is set to come before Parliament again for re-ratification in a few weeks.
Surely this is a golden opportunity for Cameron, if he really wants a referendum, to promise one? It is hard to imagine a better way to put Brown and Clegg on the spot about whether they believe the people should have a say on Lisbon or not.
Even if Cameron did not promise a referendum, in a hung Parliament he would surely struggle to get Better Off Out MPs such as Douglas Carswell and Philip Davies or even less pronounced eurosceptics to vote in favour of ratifying Lisbon without the approval of the people.
The EU Constitution has shown a remarkable ability to repeatedly rise from the grave when it had been thought to be dead. Perhaps it is now about to do the reverse.