Imagine, just imagine, that Cameron pulled this one off. Imagine that we arrived at a point where the two options on a ballot paper were a renegotiated settlement acceptable to his lot, and outright withdrawal.
It would unite the Left on the EU like nothing since a section of it first inexplicably decided that "Europe" was a bulwark against Thatcherism (several years later, Thatcher herself even more oddly seemed to begin to agree with them), much as there have always been a few people on the Old Right who have thought of it as a bulwark against Americanism.
For if the only alternative were whatever could be sold to the remains of the Conservative Party, then the only viable option would be whatever else were on offer. Namely, withdrawal.
As would then be advocated in the strongest possible terms by the whole of the Left. It would be the Thatcherites who would be campaigning to stay in. Well, of course. It was ever thus.