One reason, though probably not the most important one, is that the choice on Thursday is asymmetric. Vote Remain, and there are almost certain to be other opportunities, if we want to take them, for the UK to negotiate its way into a looser relationship with, or completely out of, the EU in the next ten or twenty years. Vote Leave, and the terms that the EU will be prepared to offer us for renewed membership at any time in the future are almost certain to be distinctly less good than the ones that we currently have.
This might not be so important for you if you think that the UK can get sufficiently good terms if it leaves the EU that we are unlikely ever to consider rejoining. However, the conduct of the referendum campaign, particularly by the Leave side, has almost certainly made this less likely. When one aims to enter negotiations with someone, then unless one knows that one is in a position of overwhelming strength (but probably not even then), it is not a good idea to make statements that might as well be designed to convince the people with whom one is going to be negotiating that one is untrustworthy or uncompromisingly hostile to them or their aims. But that is what Vote Leave (and, for that matter, Leave.EU) have been doing.
Two months ago, for instance, Michael Gove made clear that he believes that the EU's current unpopularity in some member countries as a result of the undoubted problems that the EU is currently having with the euro and with refugees mean that British withdrawal from the EU is likely to lead to these member countries following suit and starting a domino effect that leads to the EU's total collapse.
Last month, Gove explicitly stated that leaving the EU would also involve leaving (though still trading with) the single market, to allow the UK to regain control of its borders (which, as we are not in the Schengen agreement, we still largely have) and immigration.
This month, Gove has been envisaging a fairly lengthy period (at least four years) during which the UK would remain well within the single market (on effectively EEA-equivalent terms) while negotiating its way out - accepting that this would delay the fuller control of immigration that leaving the single market would at least nominally give us.
At first sight, these are contradictory in a way which one might expect of - well, Boris Johnson, for example - but not of Michael Gove. At second sight, though, they do add up - if one assumes that Gove's primary aim would not so much be an agreement as a mixture of blackmail and sabotage. The main EU leaders are quite intelligent enough to have noticed this, but also have good reason to regard their position as nothing like as dire as Gove and others in Vote Leave seem to imagine.
For instance, a comprehensive EU collapse is just about possible, but if it happens is most likely to take place over decades in a sequence of slow and messy steps - the only remotely plausible shortcut would be the election of a Front National government in France or its equivalent in Germany. But the likelihood of these possibilities would be noticeably increased if the EU conceded to the UK almost all of the access inherent in single market membership but with none of the countervailing responsibilities. And the least risky way of doing this is to make withdrawal negotiations with the UK as rapid, perfunctory and unprofitable (for the UK) as possible, consistent with article 50 of the EU treaties and wider international law - even at the cost of some limited economic discomfort for, for instance, Germany.
Because, to be clear, while the EU itself and the other 27 member countries are well aware of the EU's current problems, only a handful of the other 27 governments see the dissolution of the EU as having any possible advantages to their countries, and even the few that do see the likely disadvantages as greatly outweighing the advantages. France and Germany, in particular, are well aware that it is only the checks and balances based in the EU institutions, including the participation of other countries (the UK currently being the main, though far from only, one) that can act as arbitrators when they disagree, allow them to cohabit as neighbours in anything other than a condition of wary hostility or open war. The absence of war in western Europe since 1945 (beyond a few secessionist groups resorting to terrorist methods) can reasonably be attributed to NATO - but only if one recognises that, without the EU to provide the economic and diplomatic guarantees that French and (West) German (as well as some other) interests would not diverge widely, NATO would almost certainly have collapsed by the late 1960s, like the other regional security organisations that America tried to set up at the beginning of the Cold War (for more on this, see Anthony Beevor's article in Monday's Guardian ).
For the other 27 member countries, the UK's withdrawal from the EU would be regrettable but, particularly if it remained either in the EEA or the single market in another form, no more. The Vote Leave campaign, though, has upped the contention to the point where, if it wins tomorrow, UK withdrawal has become a hostile act against the EU and its other 27 countries - not just through Gove's remarks but through Boris Johnson's fatuous comparisons of the EU's aims to those of both Napoleon and Hitler (both of whom Britain successfully resisted for many years but neither of whom were defeated until after massively overambitious attempts to invade Russia) and through the attempts to argue that UK withdrawal from the EU would have no effect on the Common Travel Area with Ireland (only true if Ireland leaves the single market along with the UK). If either Gove and Johnson were on the UK negotiating team, negotiations would be extremely difficult and at least verging on impossible. Even if neither of them were to be, their presumed influence in the background and the distinct possibility of them or their close allies (or, even more though rather less likely, UKIP) taking over in future (and rejecting or insisting on renegotiating any withdrawal agreement even further) would make the negotiations scarcely any easier.
The UK (and before it England - incorporating or excepting Scotland) has existed in splendid isolation from Europe for very long periods in past centuries, and even survived and won wars with our continental neighbours lasting twenty years or more - because the UK was able to maintain naval (and later air) dominance of the English Channel and North Sea, at least to the extent of denying it to our opponents. While our withdrawal from the EU would be unlikely to provoke actual war in the short or even medium term, we do not seem to have the forces that would be required to maintain splendid isolation even against a determined horde of refugees (in, say, the event of EU economic collapse) without resorting to nuclear weapons (almost certainly securely double-locked by our American suppliers).
And while even a fraught withdrawal by the UK from the EU would probably not stop the UK still belonging to NATO along with remaining EU members, any consequent hostility would definitely drive a wedge between us and them. The likelihood is that NATO would weaken as both the UK and the EU felt that they should rely less on NATO for defence and look to other alliances or resources (for the EU, this could be the mythical EU army which, without far more pressure than currently exists, is unlikely even in prospect).
So voting Leave tomorrow seems to create two alternative unappealing scenarios for the UK - one where it is facing continental coasts united against (or simply ignoring) it under the EU from the Pillar of Hercules through to the Skaggerak (and, if one includes EEA Norway, on to the Russian border) and another, thankfully less likely, in which the EU disintegrates and we are left facing a continent of economic chaos and fracturing mutually hostile states, whose conflicts we have to guard against or risk getting drawn into for reasons of self-defence.
A time may well come for the UK (or some of its component parts) to decide on a looser but still amicable relationship with the European Union, or a central core of it that decides to proceed towards fuller federalisation than we are prepare to agree to - perhaps in the EEA (possibly rather revised), or some other arrangement which preserves as far as possible both membership of the single market and British freedom of action.
But, after this referendum campaign, we are unlikely to be able to get there if the UK votes Leave tomorrow.
So, vote Remain, and keep our better choices open.