WITH just over four weeks to go, there has been a weariness creeping into the Yes campaign.

Hardly surprising since it's been going full-tilt now for nearly two years. I feel exhausted and I've just been writing about it. Lots of eager volunteers have been organising conferences, producing pamphlets, joining bus tours, staging festivals. But there is only so much energy out there.

And there has also been a sense of exasperated disappointment, not so much at Alex Salmond's performance in that first TV debate - though no-one thinks it was his best outing - but at the fact that it seemed to make such an impact. Is the currency the only issue in this referendum? Can it all be reduced to the pound in your pocket?

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But the Yes campaign has to pick itself up and get back in the race - which is certainly not over. The opinion polls may appear unmoveable, but general election campaigns are only three weeks' long and a lot can happen in that time. The main problem for the Yes campaign is that, for all its energy and enthusiasm, most Scottish voters will make their decisions not on the basis of what they hear at Yestivals and Common Weal events, but through the conventional media. And here the Yes campaign receives a pasting.

Independence is portrayed, day by day, as an idea that may have some emotional appeal but is fraught with practical difficulties and unquantifiable risks. Take a look at last week's headlines: "SNP found out on oil, say Unionists"; "Carney warns of run on Scottish banks"; "Scotland stuck with Trident till 2028."

Yes, it was a typical week, dominated by negative forecasts of the impact of independence. None of them was factually incorrect, but the spin was relentlessly negative: the oil's running out; your money isn't safe if you vote Yes; Scotland's currency is toast; we're stuck with nuclear weapons on the Clyde come what may.

Well, the oil isn't running out - quite the reverse. I was in Shetland last week witnessing the new oil boom which is transforming the fortunes of the islands. There are accommodation barges clogging up Lerwick docks because there aren't enough hotel and B&B rooms to house all the workers coming in.

Yes, the short-term oil revenues are down but that is largely because the money is going into investment in the next generation.

There is, as Professor Donald MacKay put it rather colourfully last week, a "mountain of black gold" still under the North Sea. Oil & Gas UK, the industry body, puts it at between £1 trillion and £1.5trn. The idea that BP is going to opt out of this in spite at Scotland voting Yes is ludicrous.

But there isn't any secret oil discovery. There is a dash to extract more oil from the Clair field and from the newly exploited Bentley field. Indeed, some argue we should worry about the speed with which these hydrocarbons are being extracted. The Scottish Government is rather too keen on tax incentives to hasten extraction, in order to make Scotland look wealthier. This clashes with its professed concerns about carbon emissions.

So the oil is there but some economists continue to argue Scotland would be an economic basketcase even so. It was widely reported that the Bank of England Governor is making contingency plans to cope with "capital flight" after a Yes vote. Actually, since the biggest Scottish banks, Royal Bank of Scotland and Lloyds, are also England's biggest banks, many people would be taking their money out and putting it into the same bank.

Capital flight is a theoretical possibility. It happened between Slovakia and the Czech Republic after their "velvet divorce" in 1992. But this was largely because Slovakia was a relatively backward economy compared with the more industrialised Czechs. As the BoE's Mark Carney said in his speech in Scotland in January, Scotland and England are, in many ways, an optimal currency zone, provided Scots allowed a "ceding of sovereignty" on interest rates and borrowing.

The threat of bank runs is a by-product of the UK's threat to wreck the Scottish economy by denying it use of sterling. This is the "rough wooing" that lies behind the mawkish love-bombing by UK celebrities. The currency diktat is destructive and irresponsible. English firms exporting to Scotland will have to pay to change currency, as will people visiting their relatives. Firms seeking to invest in Scotland are forced to put plans on hold because of the talk of bank runs and currency blockades.

In fact, we learned last week that Carney has indeed been in "technical discussions" with the Scottish Government about currency options. It would be irresponsible for the Bank of England not to be. However, we were told the Scottish

Finance Secretary, John Swinney, had been rebuked by the BoE for blowing the gaff. The bank said it had only been answering "technical questions" and had not contemplated any currency union. The worst possible spin was put on this story, as if Swinney had been caught telling fibs.

As for Trident, we heard last week that it could be removed to Plymouth Devonport at relatively little cost. The Royal United Services Institution - a think-tank close to the Ministry of Defence - said it could cost £3.5 billion, not the £20bn floated by former UK Defence Secretary Philip Hammond. However, it raised the question of whether Plymouth residents would accept the risk of "accidental ignition" - a risk Glasgow has lived with for 60 years. My suspicion is that Plymouth would not be so sanguine and this weapon of mass destruction would be quietly laid to rest.

The press didn't seem quite sure whether this was a good story or a bad one - rather like the immigration statistics, which showed a dramatic increase in immigration from England, but also suggested Scotland might need another 14,000 immigrants in future to meet the Scottish Government's economic targets. Right-wing papers such as the Daily Telegraph presented this influx as a threat; others hardly reported it. In fact, the Scottish population is higher than it has ever been, confounding forecasts from seven years ago of terminal decline. The threat, if anything, is UK immigration policy, and Labour's threat to set up border posts at Gretna to stop immigrants coming through Scotland to England.

Perhaps the Yes campaign has to get off the narrow economic arguments and make a moral case for independence. That it isn't about Scotland swimming in oil, but about it being one that makes a moral stand against weapons of mass destruction and for free movement and open borders. It's unusual for nationalist movements to be keen on disarmament, immigration, ethnic diversity and gay rights. They should make the most of it. There is, in the end, no answer to the currency question because the UK has refused to negotiate. So let the media obsess about the pound … and accentuate the positive.

Iain Macwhirter will chair the Sunday Herald/Bloody Scotland debate on Scotland's post-referendum future in Stirling on Saturday, September 20.