I guess Dave is hoping that the other parties will opt to be anti-referendum, which would further shore-up the core Tory vote, or would it. The really interesting scenarios arise from ones prognosis of the Euro negotiations, if they go badly Dave has signaled that he will campaign for withdrawal and this quite likely. So here are some of these scenarios modeled out. Maybe Dave is betting on a poor deal from Europe and the freedom to campaign for withdrawal, because this looks his most likely road to success.
When Harold Wilson tried this referendum trick in the '70ies he managed to con the country that his 'new deal' was substantially better - when it was anything but! Dave will be under much greater scrutiny so it will be very difficult to prove that the new deal is different, but countered against this a number of Europeans will be desperate for us to stay in! So let the fun begin!