Between 1707 and about the mid-1930s, Great Britain was the most successful and important nation in the world. In a fortnight, it may no longer exist.

Until last month, that was unthinkable. Now it may even be likely. What the hell just happened?

The Union of Scotland with England (and Wales) was in 1707; 1934 was the foundation of the Scottish National Party. For most of the last 80 years, no one cared about that, because it consisted of three nutters and a dog, all wearing kilts. Scottish independence was as likely as the resurrection of all those tiny European countries from before 1914.

Even when those tiny states did pop up again over the past couple of decades, it was still far less likely. Yuogslavs always hated each other, but in Britain the rivalry didn’t extend much beyond football.

The SNP did a bit better in the 1970s when Scotland struck oil – thanks to their brilliant slogan “It’s Scotland’s Oil!” – but even then never got much more than 20 percent of the vote and a handful of MPs.

When Scotland got devolution from Westminster at the end of the last century, it was assumed that it would kill nationalism stone dead. Even when the SNP became the government, there were a lot of SNP voters who wouldn’t have voted for independence.

They’re being asked to do that on September 18. But during the run-up, which has been going on for the best part of three years, the result has been a foregone conclusion. As recently as the beginning of last month, a YouGov poll (excluding Don’t Knows) put support for No at 61 percent, with the Yes camp trailing badly at 39 percent. The bookies were offering 5/1 on a Yes vote.

The voters (and bookies) are now much less sure. By Monday [Sept 1], with just over a fortnight to go before the poll itself, the same organisation found 47 percent intending to vote Yes, with the No campaign only just ahead on 53 percent.

Many Scots who want to stay in the UK, and the rest of the UK (rUK, in the referendum jargon) and who hadn’t paid much attention to the campaign are waking up to the possibility of separation, and wondering whether the No campaign has been sleepwalking towards the end of their country.

Trying to explain the Nationalist surge, many blame the poor performance of Alistair “Better Together” Darling, of the No campaign, in the recent, second televised debate against Scotland’s First Minister, Alex Salmond. But though most agreed Mr Salmond won it, Mr Darling trounced the Scottish National Party’s leader in the first debate.

In that meeting, the No campaign scored with its principle line of attack, mocking the Nationalist leader’s inability to say what would happen if the rUK refused to enter a currency union. But despite all the main parties – and most UK voters – insisting that would be the case, it doesn’t now seem to put many Scots off. Nor do questions about whether oil can pay for everything the SNP is promising.

Mr Salmond’s side has insisted these are minor details, and will all be worked out. Instead, they’ve been concentrating on “Tory threats to the NHS” – despite the fact that the Scottish health service has been fully devolved since 1999, and the only person who actually has the power to privatise it is Salmond himself.

If that strikes you as a bit brazen, you need to see a bit more of Mr Salmond in action. It turns out that the SNP Government is doing just that: Greater Glasgow’s NHS, under the Scottish Government’s contracts scheme, has just given WeightWatchers the job of dealing with its fat patients – a lucrative gig in the country which introduced the world to the deep-fried Mars Bar.

None of that seems to matter. Mr Salmond’s claims have gone down well with voters who regard the NHS, and other public sector services, with almost religious adoration. Though Scotland produced Adam Smith in the 18th Century, and the most entrepreneurial inventors and manufacturers the world has ever seen in the 19th, for most of the last century the country has been well to the left of the rest of the UK. Of its 59 Westminster MPs, just one is a Conservative. They have, as they like pointing out, more giant pandas than Tories.

The Scottish Parliament at Holyrood already has considerable powers. They even include the ability to vary income tax (which has never been exercised). If anything, though, this seems to have increased many Scots’ desire to distance themselves from Westminster.

The Yes campaign’s late surge has probably been the result of playing on just such contradictions. As you might expect, they have been talking up the possibilities of independence would bring – if they don’t quite claim that the rivers will run with whisky, the North Sea is still full of black, black oil, and almost everything is bound to be much better.

But, cannily, they’ve also been keen to argue that the things that Scots currently like about the Union – sterling, the Queen, the BBC, the NHS, state pensions and EU membership – will remain basically unchanged.

Better Together, on the other hand, has been accused of waging an entirely negative campaign – something that, to be fair, it’s difficult to avoid when you’re trying to persuade people to vote No.

They can’t really argue that Scotland couldn’t be independent; first, because it would annoy their audience, and secondly because there are lots of countries its size which do fairly well. And it’s an uphill struggle to claim the status quo is an improvement when the argument for independence is that it will sort out all the problems.

What’s more, Nationalists run an impressive campaign machine; they are a vocal – some would say bullying – presence online. At the last elections for Holyrood, the SNP won a majority despite a voting system specifically designed to prevent any one party from doing so. And in government, it has been considered generally efficient and effective by many Scots who – at least until recently – would have opposed separation.

Whether the Yes camp have been optimistic and evasive, or the No campaign scaremongering and complacent, there’s one obvious psychological reason for the recent swing in the polls. Conversion is almost entirely one-way. Anyone who begins to lean towards a Yes vote is, by definition, accepting the case for a huge and potentially risky change.

The Nationalists’ advantage is that the future can always be better, because you can change it. The advantage of the past and present – that you know where you are with it, is a much harder sell.

As a result those who have been converted are almost impossible for the Unionists to win back. It doesn’t matter how convincing the arguments about the dangers and unknown quantities are; they have already made one leap of faith, and after that all the practicalities become details.

If the campaign is as tight as the polls now suggest, it will be Scotland’s undecided voters who determine the outcome. The indications may still be – just – that they will choose to remain in a United Kingdom, but there’s now a real chance that those still wavering will be convinced by momentum. If they seriously consider voting Yes, the No campaign’s arguments – however solid – become irrelevant.

All this also guarantees that the divisions in Scottish politics will not end at the referendum. A Yes vote would, of course, be the most significant event in British domestic politics for centuries, but half the country will be stunned to find themselves with a new nationality.

A No vote won’t settle anything either, though. All the main Westminster parties now say they will re-examine Scotland’s constitutional arrangements. Meanwhile, those who think that only independence will do won’t abandon their campaign. And the narrower the margin, the more zealous they’ll be.