Scottish referendum: this is the hardest campaign I have ever had to call

The ballot paper simply asks: "Should Scotland be an independent country?" That is the beauty of a referendum – and the brutality. Complexity is mercilessly reduced to the binary. Yes or no? The kaleidoscopic relationship between the nations of these islands; all those decades of rivalry, friendly and not so friendly; all those centuries mingling blood and ideas and people; all that rich intricacy is boiled down to one stark choice.

Yet they are anything but simple, the arguments swirling through Scotland and beginning to gust into England at the climax of the campaign. Behind the one on the ballot paper is a thicket of other questions, larger even than the destiny of five million Scots and the fate of the rest of the UK. Is there a future for the idea of multinational union, the concept which this island rather accidentally pioneered when two thrones were united under a Stuart king called James and two peoples were then combined under his great granddaughter Anne? In the turbulent opening decades of the early 21st century, is smaller and separate smarter, or is it cleverer to pool resources and seek safety in numbers? Does the politics of identity now trump the politics of ideology and are both struggling not to be overwhelmed by the anti-politics of a visceral contempt for the very business of governing?

There are people who have emphatic answers. Standing on the Irn-Bru crate which he is taking around 100 public squares, Jim Murphy, the Labour martyr to nationalist egg-throwers, who wears the splatter on his jacket like battle honours, speaks for the union with an eloquent intensity as he decries the independence campaign as purveyors of false hope. "Changing your passport," he says, "never put a penny in anyone's pocket." From the other side come nationalist voices burning with matching emotion as they argue that Scotland must seize a "once in a generation" opportunity, not just for its own sake, but to be a beacon for how government can be better and society fairer.

It makes for great theatre on the street and electric engagements on TV, but these clashes are not entirely representative of what is going on. For I have also met plenty of folk who regret this brutal polarity. One such is Richard Holloway, a former bishop of Edinburgh and chairman of the Scottish Arts Council. He makes a terrific case for Scotland remaining part of a radically decentralised, federated UK, but he can't vote for "devo max": it is not on the ballot paper. Aggrieved at being forced to choose between "rubber-stamping the status quo" and independence, he will vote for the latter as a self-described "anguished yes".

Then there are the "agonised noes", as I would describe the woman who told me she thought the status quo rotten but independence too risky. So she would vote against it, but with the fear that Westminster would respond with a big sigh of relief swiftly followed by "a return to business as usual". At an Edinburgh community centre, one woman said: "There seems to be daily pressure on me to decide who I am."

The man who is doing the pressing is Alex Salmond. This referendum wouldn't be happening without his tenacious pursuit of the cause through all the years when it looked hopeless, his cunning as a tactician and his skills as a campaigner. He celebrated his 10th anniversary as SNP leader by going walkabout in the centre of Glasgow with his deputy, Nicola Sturgeon. They were accompanied by a cheering claque of supporters. The nationalists have always had the advantage when it comes to generating public displays of energy and excitement. They have also been better organised. Back in January, I attended a dinner with the SNP leader at which he gloated that he had caught the no camp napping by bulk-buying billboard sites for the campaign endgame. Scotland will see the results on Monday, when the country is plastered with yes posters.

It is at the heart of nationalist strategy to project independence as an idea whose time has come, a historical inevitability that can no longer be resisted. The recent poll surges in their favour give them momentum, a precious commodity in the final furlong. If voting for independence is a leap of faith, a gathering sense that sentiment is swinging that way has the potential to be a self-fulfilling prophecy by emboldening more Scots to make the leap. Shivers of fear are now running up the spines of the UK-wide parties as they wake up to the possibility that the secession of Scotland could be under way in less than a fortnight. Tories are talking about David Cameron being forced to resign and Labour people are having fits of the vapours. In the words of one senior Labour figure: "It is squeaky bum time."

There are Scots who feel passionately attached to the union. One man buttonholed me on Edinburgh's Grassmarket. He wanted to argue with the contention of Tom Devine, the pre-eminent historian of Scotland, that the union might as well be dissolved because all that England and Scotland have left in common is history and family. "Even if that were true," he said, "those things matter." Labour's Douglas Alexander, not a politician who usually trades in emotion, tells me he would feel "a profound sense of grievance and loss if I were deprived of the British side of my identity".

Where Better Together has struggled is in shaping those feelings into a message that makes the union sound like something worth fighting for. Alistair Darling was penetrating on the fiscal hazards of independence in his TV debates with Alex Salmond, but the former chancellor struggled when it came to articulating a positive vision of the future of the union. His worst moment was when a member of the audience asked: "You say we're better together. What's better together about now?" Shoogly is a good Scottish word to describe the look on his face as he struggled to formulate a response.

Andrew Wilson, a former MSP who is one of the most interesting exponents of independence, characterises the relationship between Scotland and England, by which I think he mainly means London, as a corrupted bargain: "You send us your talented people and resources, we send you the methadone of public subsidy." Not many other nationalists would put it quite like that, but a lot of Scots nod along with the basic contention that the relationship is bust.

Salmond also has the best line – best in the sense that it is hard for his opponents to counter. "No one will run the affairs of this country better than the people who live in Scotland." His cause has also been given rocket fuel by contemporary discontents. As one leading figure in the no campaign says: "The last five years have been shite for many people and they are looking for someone to blame."

A regular theme of Salmond's speeches is to lay all of Scotland's ills at the door of "the Westminster elite", the same trope that Nigel Farage uses so effectively to some English audiences. Among reasons to go independent that I have heard from yes voters were new discontents such as the bedroom tax, and older reasons to be angry such as the Iraq war, along with quite a lot of mentions of the expenses scandal. The nationalists have also been adept, if rather deceptive, at presenting independence as the miracle cure. Austerity, corrupt politicians, remote politicians, elitist politicians, the consequences of globalisation – all will evaporate if only Scotland releases itself from the choking yoke of an over-weening London. And – which is true – they'd never again be governed by Tory prime ministers that Scots didn't vote for. Salmond can inveigh against Margaret Thatcher and the poll tax to applause from people far too young to have been alive in the 1980s.

There is an irony here. As Holloway remarks: "The Scots are, in many ways, quite a conservative people." A likely upshot of independence would be the revival of the centre-right as a political force. Salmond himself has been heard to say so. But today it is almost culturally illegitimate to be a Tory in Scotland. The most helpful thing the leader of the Scottish Tories has done for the cause of the union is to say that it "isn't looking likely" that Cameron will win the next general election.

Both sides agree that the outcome now hinges on the behaviour of those voters who have been traditionally Labour. That is why 100 Labour MPs will be north of the Tweed this week, among them the party's "big hitters", with the message that Scots don't have to vote for independence to be rid of the Tories because Labour will win the next UK election. They will seek to amplify Ed Miliband's message: "We can build a more just Scotland within a more just United Kingdom." The push will culminate with a rally that the Labour leader will front alongside Gordon Brown.

How well this will work – telling Scots to spurn independence because the Labour cavalry is galloping over the hill – is a bit moot. It asks Scots to trust in Labour's rather slender national opinion poll lead. Another problem is that Labour is not held in great affection in Scotland anyway. If it were, the nationalists would not have won the last two elections to the Holyrood parliament.

Where the nationalists are still on very boggy ground is convincingly describing the sequel: what would happen the morning after Scotland woke up to find itself independent. The two sides have now shelled each other with so many rival claims about oil revenues, currency and borrowing that they have numbed each other and probably the electorate as well. What tells is that the only tax cut that Salmond has promised is one which will be of most benefit to large companies: an independent Scotland would set its corporation tax rate at 3p in the pound less than George Osborne. On his Glasgow walkabout, the SNP leader was stalked by no campaigners baiting him with placards bearing the slogan: "Tax cuts for the rich!".

Whatever he says about job creation, making a priority of handing more money to multinationals sounds like a funny way of laying the foundations of a more egalitarian country. The big hole in the nationalist prospectus is that it promises Scots that they can have Scandinavian standards of public services with American levels of tax.

The other disingenuous element of their case is about sovereignty itself. An independent Scotland would obviously be free to make more choices about its future: gone would be the Trident nuclear subs. But many of its choices would still be constricted within parameters set by major external forces. Those forces would include London, a city with more people and money than the whole of Scotland put together. If Scotland did somehow manage to retain the use of the pound, its interest rates would be set by a bank governor and a monetary policy committee appointed by a Westminster chancellor. If it was readmitted into the European Union – which, after some aggro, I expect it would be – an independent Scotland would have to negotiate many of its choices through Brussels. The value of its oil and gas would be determined by decisions made in Riyadh, Tehran, Moscow and Beijing. The cost of its borrowing would be set by bond traders in New York and Frankfurt – and, yes, those evil bloodsuckers who live and work in London, quite a lot of them Scots.

Small can be more nimble, more buccaneering, more prosperous. Some pro-independence business leaders say that part of the appeal is that John Swinney, Scotland's finance minister, is easier to get on the phone than Osborne or Ed Balls. This cuts both ways. Small can also be more vulnerable to corporate pressure. The Swiss-headquartered owners of Grangemouth, the vast refinery on the Firth of Forth, are already very influential actors in Scotland. They would be even more powerful in an independent Scotland.

From the no side, people tell me that they think the best card left for them to play is the argument that a canny Scotland would seek to exploit "the best of both worlds" by gaining more freedom to govern itself within the union, without taking the gamble of going it alone in an insecure world. "Change without risk," is how it is described. All the main UK-wide parties have now signed up to versions of "devo max"; where they've so far failed is convincing enough Scots that they are sincere about it.

Of the many campaigns I have covered, this is one of the most tricky to call. The hard-to-predict elements include "the missing million": the people who never or rarely vote at an election, but who are expected to come out in large numbers for the referendum. The nationalists think that ought to help them and I am told the first returns from postal voting suggest an extremely high turnout. Another factor hard to gauge is the number of "shy noes" – Scots too embarrassed to say that they are going to vote against independence for fear of being seen as unpatriotic. The opinion pollsters are very twitchy about getting this one wrong.

Whatever the result, it won't be easy for anyone to plausibly claim that the outcome represents a settled consensus of Scottish public opinion. Scotland will have spoken, but she will not speak with an unequivocal voice. Which is why you hear Scottish civic leaders expressing foreboding that it will be hard for wounds to heal and for the losing side to swallow defeat. Edinburgh's poet laureate, Christine De Luca, has even penned an ode to reconciliation called The Morning After.

There was a position that would have gathered a very solid consensus behind it. Had it been offered as a choice, I am certain there would have been a thumping majority for "devo max". From that, I conclude this. If Scotland votes to terminate the union, a generation of Westminster politicians will have to reflect in their post-resignation memoirs on why they were so slow to see it coming and too late in shaping a response. If Scotland gives the union another chance, a constitutional reconfiguration will have to be pursued with conviction and urgency. And it can't stop at Scotland: for England also has her discontents with the union. The very concept of a United Kingdom has never before faced such a test.