The doomsday scenario here is a narrow win for remain.
The reason is simple and the polls back this up.
Those who want to leave the EU are a committed bunch. Indeed the pollsters have to reduce their weighting because the vast majority say they are certain to vote.
Hence if this once in a double generation event goes against them they will only have one resort and that is voting UKIP in 2020.
Cameron was very foolish in trying to rig things in his favour. £9 million of taxpayers money on leaflets, denying access to the civil service such that proper economic forecasts could be made and accepting huge sums from rich bankers with vested interests like JP Morgan and Goldman Sachs was not bright in my opinion.
Ukip now has an MP, and therefore a whip too. This is important as it means that MPs can defect, there was a chap who tried to before an MP had been elected and was told that he couldn't.
Quite how many disenfranchised MPs from all parties would actually do this is a matter for speculation however the dodgy antics of the government won't have helped.
It wouldn't take many of the 100+ eurosceptics to cross the floor for Cameron to lose his majority and they would spend the next 4 years campaigning for UKIP in 2020.
How many of the 45%+ of the electorate would vote UKIP in 2020 as the only way to extract us from the EU?
Well the only reasonable answer is a lot more than voted for them last year. Add in the dodgy expenses that the tories submitted mainly in seats where they were trying to beat UKIP and re-runs of these elections could see even more UKIP MPs.
Oddly enough the best possible result for Nigel Farage is to lose the referendum....