Anything could still happen in Thursday's referendum, but if Leave wins, Britain will exit the EU. Or will it?
Suppose that come Friday breakfast-time, Leave has prevailed. David Cameron has always insisted that despite his presumably profound misgivings he would proceed to arrange withdrawal. If he were toppled in the coup that many expect, his successor, given the attitudes of Tory party members, would almost certainly be a committed Brexiteer.
Either way, a British prime minister would begin the discussions with our European partners about the course of our departure required under article 50 of the Lisbon treaty. Once these were complete, negotiations would begin about our future relationship with the EU. We might or might not end up within the single market or the European economic area, but within a fixed period of two years the UK would be out of the EU, unless the 27 chose to allow us to extend this term. Or so you might suppose.
The royal prerogative certainly empowers Her Majesty's ministers to undertake treaty negotiations on behalf of the nation. The prime minister would be perfectly entitled to embark on the process of extracting us from the Treaty of Rome, Maastricht and the tangle of other agreements associated with them. Yet he would face a problem.
Since the 17th century national sovereignty has resided not with the crown and its servants but with the parliament at Westminster. Ministers might extract the country from the EU's institutions but it will remain subject to the provisions of Brussels until the European Communities Act 1972 has been repealed.
The government would have to introduce a bill to bring this about. In practice, it would also have to bring in enabling legislation to empower itself to unravel a vast panoply of associated embroilments. The Constitutional Reform and Governance Act 2010 requires that the withdrawal agreement negotiated with our EU partners would itself have to be laid before parliament, which would have the power to delay ratification indefinitely.
Normally, a government relies on its parliamentary majority to carry through such proposals. However, the Brexit question has shattered normal party allegiances, and they seem unlikely to reassert themselves as long as the issue that destroyed them continues to tower over the nation.
Leave might have won the referendum, but at least half of Conservative MPs have been backing Remain. The vast bulk of Opposition MPs have done likewise. Overall, the Commons seems to be at very least 70 per cent pro-Remain. The Lords are of course even more pro-Remain.
If Leave's referendum victory has been decisive, this may not matter. The irrefutable will of the people would doubtless prove irresistible. However, though the polls cannot tell us which side will win, they do seem to have established something: the result is fairly certain to be close.
This will mean that those who do not like that result will be able to present it not so much as a defeat but more as a tie. What the populace has really proclaimed, they may suggest, is that it cannot make up its mind. Suppose that 52 per cent vote Leave on a turnout of 64 per cent (which was the turnout in the referendum on EEC membership in 1975). It will be possible to argue that support for withdrawal has been demonstrated by barely a third of the electorate. And that figure might have been even lower if predominantly pro-Remain groups like the under-18s and long-standing expats had not been denied a vote.
For parliamentarians, this point will not be academic. In some jurisdictions, referendums are binding. This is the case with those held regularly at federal, cantonal, and municipal level in Switzerland. However, because the British constitution makes parliament sovereign, a referendum in the UK can only be consultative. The government pamphlet explaining the 1975 referendum spelt out that "the British Parliament in Westminster retains the final right to repeal the Act which took us into the Market on January 1, 1973."
David Cameron and his Cabinet colleagues may have committed themselves to implementing the verdict of the current referendum, but parliament has not. The act that authorised the 2011 referendum on the alternative vote obliged the government to implement electoral reform if the Yes side won. The legislation enacting the EU referendum, on the other hand, contains no such requirement.
So it would be down to parliamentarians to decide whether or not Brexit should go ahead. They might well feel this presented them with not just an opportunity but an obligation. We have opted for representative rather than direct democracy in the belief that the populace is not as well suited to determining complex issues as people selected and equipped for the job. Why become an MP if you're going to duck such a mighty responsibility as this one? After all, the public have informed us in vox pop after vox pop that they don't understand the subject on which they will have voted.
As parliament mulled the matter, it would come under great pressure to block Brexit not just from all corners of the British establishment but from international organisations and Britain's allies overseas. The EU would doubtless be offering blandishments, perhaps even new concessions, and implying that it would reform itself.
Even Leave campaigners accept that the markets would be in turmoil. Sterling would fall and prices would go up. Interest rates would then rise too, pushing up rents and triggering a wave of mortgage defaults and repossessions. Opinion polls might well be suggesting that a majority of the public was now eager to repent of the course on which the referendum verdict had set it.
To snuff out Brexit, all it would take would be the votes and abstentions of enough parliamentarians as the government's legislation tried to worm its way through Westminster's tortuous processes. Amendments might perhaps be added that reversed the purposes of bills.
Faced with such obstruction, the prime minister might seek a general election, if only to secure his own release from the nightmare in which he would find himself. Unfortunately the Fixed-Term Parliaments Act of 2011 would prevent him from just asking the Queen for a dissolution. Nowadays, parliament can only be dissolved before its term is up if the house of commons votes for an election by a two-thirds majority or the government loses a confidence vote. That means the decision would be up to parliament itself.
Why would MPs renounce their stranglehold over the fate of the nation at such an important moment in our island story? The government would surely find that its only practicable course would be to bow to the will of our sovereign parliament and abandon Brexit.
Of course, not all of the above would necessarily come to pass. As soon as it was apparent that this was the path on which the nation had embarked, reality would probably assert itself. In fact, the government might simply announce quite quickly that article 50 would not after all be invoked: unfortunately, parliament had over-ruled the executive, as was its constitutional right. The Brexiteers would be awkwardly placed to protest, since, ironically, the sovereignty of the UK parliament is supposed to be what they're fighting for.
The dust would settle, and Britain's abortive flirtation with EU exit would take its place in the annals of Brussels alongside Denmark's short-lived referendum No to Maastricht, Ireland's overturned rejection of the Treaty of Nice, the elegantly side-stepped French and Dutch thumbs-down to the European constitution, Ireland's quickly rescinded veto of the Lisbon Treaty and the Greek reverse-ferret on its bailout terms.
A vote to Leave on Thursday would create a lot of fuss. But it might prove less momentous than expected.