It seems to have come as a shock to many a journalist that people’s willingness to back independence might turn on whether they reckoned it would leave them better off or not.

Surely nobody would be willing to sell their right to be British for as little as £500 a year, as suggested by the latest figures from ScotCen’s Scottish Social Attitudes Survey released earlier this month?

Well, of course, hypothetical survey questions are just that – hypothetical. We will never know whether 65 per cent would actually vote for independence if they thought everyone would on average be £500 a year better off as a result – not least because there will never be consensus amongst the Scottish public as to what the financial consequences of independence would be.

Yet behind the headline there was a crucial truth – perceptions of the economic consequences of independence are already playing a crucial role in shaping people’s attitudes towards Scotland’s constitutional future. At present, relatively few voters are keen on independence unless they are unconvinced that it would deliver an economic benefit. So while the constitutional debate is, of course, partly about the heart and feelings of national identity and pride, it is also very much about the head too.

Those who say they are Scottish and not British are, as we would expect, far more likely to back independence than anyone else.

As many as 53 per cent currently do so, a considerably higher figure than the 32 per cent of all Scots who belong to that camp. Yet this equally means that nearly half of those who deny any semblance of Britishness would apparently still prefer to be part of the British state, while less than half, 43 per cent, actually feel confident about the prospect of independence.

The mood amongst those who say they think Scotland’s economy would be ‘a lot better’ under independence is much more enthusiastic than this. No less than 78 per cent of this group currently back independence, while exactly the same proportion also feel confident about the prospect of independence. These economic optimists even outstrip those who think that independence would strengthen Scotland’s voice in the world in their enthusiasm for independence.

Unfortunately for Mr Salmond, at present far fewer people reckon Scotland’s economy would be a lot better under independence than deny any kind of British identity. But if the First Minister could narrow that gap, it seems that support for independence could easily grow.

In truth, none of this ought to come as a surprise to unionists. From ‘divorce is an expensive business’ through to the endless debate about the state of Scotland’s public finances and more recently, claims that Scotland would never have been able to rescue its banks, the argument that independence would leave Scotland worse off has arguably been the central plank of the unionist cause for much of the last decade.

In contrast, unionist appeals to Britishness or attempts to wrap themselves in the Union Flag have, if anything, been noticeable by their absence.

And herein perhaps lies the real rub for unionists in the figures from the social attitudes survey – their economic arguments have less force than they imagine. While only 10 per cent reckon that the economy would be ‘a lot better’ under independence, and no more than another 25 per cent ‘a little better’, equally, only 29 per cent feel that Scotland’s economy would be ‘a lot’ or ‘a little’ worse under independence. That seems like a pretty poor return for many years of claims that independence would be an economic disaster.

Indeed, given how evenly balanced opinion on the economic consequences of independence is, it might seem surprising that support for the nationalist project is not higher. Here we come to another crucial survey finding. Fortunately for unionists, those who are unconvinced by the economic arguments of both sides are currently still inclined to worry about the risks of independence and thus to stick with the Union.

But that looks like rather a thin reed on which to rest the future of the British state. One certainly may wonder whether it would prove sufficiently strong in the wake of an ineffective unionist campaign – and that is a fate that it seems we cannot be sure will yet be avoided.

To be effective, political campaigns require one attribute above all – unity. However, this looks to be in rather short supply within the unionist camp.

First of all, unionism is riddled with traditional enmities. In their attempts to appeal to their party faithful, the Scottish Labour leadership candidates have all signalled a reluctance to share a referendum campaign platform with the Conservatives. It would not be surprising if they took the same attitude towards the Tories’ UK coalition partners, the Liberal Democrats, too.

It is all very redolent of Ed Miliband’s reluctance to share a platform with Nick Clegg during the AV referendum in May – and much good it did that cause. In contrast, the No to AV camp showed remarkable cohesion – and there seems little doubt that in the Scottish campaign the pro-independence camp will put on a good display of unity and cohesion too.

Meanwhile, apart from frictions between personalities, the unionist camp also seems at risk of failing to agree on the vision of the Union to put before the voters. The consensus that was once generated by the Calman Commission seems be breaking down. Ruth Davidson has declared that Calman is a line in the sand that should not be crossed, Willie Rennie in contrast thinks it is time to consider ‘Home Rule’. Meanwhile Labour seems to be debating amongst itself what it should now do.

And if unionists cannot agree on what the future shape of the Union should be, then what will those crucial voters who currently are unsure of the economic consequences of independence begin to think? Will they still reckon that the Union is the safer, less worrisome option? Or might independence begin to look the better bet after all?