The only way he can avoid a political epitaph no-one wants, especially not a Tory PM, is to win an EU referendum. Mr Cameron himself does not want Britain to leave the EU, but he has repeatedly thwarted himself in this aim.
Firstly, he hasn't stood up to (never mind stamped down on) the Eurosceptics in his own party, whose only acceptable version of the EU would be a kind of British empire mark two where other countries agreed to everything Britain wanted.
Secondly, by pulling the Tories out of the EPP, the main centre right political group in the European Parliament, Mr Cameron left the largest group, weakened it and lost close cooperation with ruling centre right parties like that of German Chancellor Merkel and Spanish PM Rajoy. That's not ideal when you need to their help with your "renegotiation".
Thirdly, by repeatedly saying (to placate Tory Eurosceptics) that he wants to "renegotiate Britain's membership of the EU" rather than seeking EU reform that bring benefits to all including Britain, he has got other countries' backs up immediately. Why should the UK (or any individual EU country) get special conditions? If one can have special conditions, why not all?
It is however worth pointing out that contrary to popular belief (at least in the UK), EU legislation isn't "one size fits all", but almost all of it contains tweaks here and there to suit individual countries, as long as this doesn't negatively impact others. That is not what Mr Cameron's backbenchers mean. Those who don't want a British empire mark two, want to cherry pick EU rules instead, no doubt jettisoning ones that protect the environment or worker's rights, while keeping those that help British companies win business across the EU.
So, for the toughest battle of his life, Mr Cameron has decided to metaphorically tie his hands behind his back and blindfold himself.
Then there's the thorny question of what would happen if the UK as a whole voted to leave the EU, but some home nations voted to stay. The Celtic nations, in particular Scotland, are far more pro European than England. SNP leader Nicola Sturgeon has already said that an EU vote where the country as a whole voted to leave, but Scotland voted to stay, would pave the way for a second independence referendum and Plaid Cymru leader Leanne Woods has said that a decision to leave the EU should be one voted for by all UK nations.
Surely a second Independence referendum in Scotland on the back of an "England is forcing Scotland to leave the EU against the will of Scottish voters" message would stand a far greater chance of success? I would imagine Mr Cameron does not want to take such a chance....
