Every indication is that David Cameron is pulling forward a European vote - negotiations are immediate and then a whether to stay or go in a referendum can take place.
This is rapid timing, probably 2016 instead of 2017, not because he inhabits a glow of approval by the electorate (that's unclear), but because he inhabits a glow of approval among his MPs. Nevertheless, the quicker the vote, the less such a referendum becomes a vote for or against him as such (as the AV referendum was a vote in part against Nick Clegg and his betrayal of his party's electorate and telling the truth).
None of this can take away the predicament generated by having a vote: this is the UKIP opportunity, and this is the Tory nutjobs opportunity. A vote to stay in would be indeed generational, and they will go ballistic if this happens. A vote to come out will be seen as highly destructive, and won't work with the condition of United Kingdom devolution: say bye to Scotland, Wales will be bitter, Northern Ireland cannot be out if Ireland is in (and modernising).
What Cameron needs from the EU Commission is some bones he can produce as something worth accepting: he needs a 'victory' of negotiations. Because he needs these, the Council of Ministers may not be as keen to offer as much as the Commission. And as everyone should know, but don't because ignorance has been part of our EU problem, it is the Council of Ministers that makes the decisions. If is a confederation of Executive arms of governments of Member States.
He knows that there is a small majority, if fluid, to stay in. So it has always been a party gamble, and the one that checked UKIP as taking Conservative votes whereas UKIP did take Labour votes - the argument more about immigration of course.
What Cameron may get before a referendum Miliband could have got without one: nevertheless, post the election now is not the time for Labour to jump on board. The referendum is to be a reality. This means Labour has to recommend a stay in vote. But it should do so without approving the referendum itself. However, it is changing its stance.
Bad decision (wrong time, wrong decision).
When the shit hits the fan the Tories will stay Labour was in favour of the referendum and should share the blame. Now it is not as bad as actually having the policy and doing it. The Conservatives tried to say Labour was in favour of the ERM when we were forced out - Major's government lost economic credibility from then on. To the victor the spoilheap. Nevertheless, it is far worse if it is exclusively your policy.
Let's be clear. If we stay in - the most likely result whether Cameron says he has enough or says (will he?) he hasn't - the Conservative majority of 12 will vanish. The bitterness in Tory ranks will be clear. UKIP will represent the neverendum that is given to the Scottish Nationalist position.
Coming out will be utterly destructive: you cannot devolve membership of the European Union. The SNP stance - and they could take the Lib Dem hold in a liar's by-election sooner than later - is one that simply represents Scotland. Their no matters, and again they and the rump Lib Dems favour a referendum only with a further transfer of powers to shared sovereignty.
A 'This Union or That Union' vote, when there are a few bones for Cameron, is surely going to be to keep This Union. Then let the Tories lose their majority, by indiscipline, by some others joining Carswell.
Labour should approve a no vote, but stay with the original formula of when a referendum is relevant. It's current policy shift to approve a referendum is a mistake. Cameron has got what he wished for, and bringing it forward won't make a difference once the reality is laid out. He'll end up wishing he had a coalition partner. He might have another party to share the blame.