This isn't an in out referendum on Europe, but an out out referendum. The PM says he will try to negotiate a new deal with Europe and put that to the people in a referendum. But he must know that the kind of package of repatriated powers that he is seeking, or rather his Tory backbenchers are seeking, is impossible because it would not be compatible with Britain's continued membership. Social and employment laws, business regulations, criminal justice and human rights. There's just no way that opt outs on all this will be acceptable to other member states.
So, what happens when he comes back from Brussels with an empty briefcase? He puts the agreement he can't get to the British people in a referendum? There wouldn't be anything to vote on. He would proposing that Britain rejects the proposals from the EU. There would be nothing to vote Yes for.
What Cameron has tried to do is neutralise the threat from UKIP. But in doing so he has effectively set Britain on a course which could only lead to departure from the European Union.