Alex Salmond’s belief that independence will be achieved on the back of a “rising tide of expectations” is drawn from recent Scottish political history. It’s no coincidence that support for the SNP boomed in the 1970s following the discovery of oil and gas in the North Sea then slumped in the 1980s as the UK economy entered a severe downturn.

The near doubling of Scottish rates of poverty and unemployment during the Thatcher era sapped Scotland’s economic confidence, reinforcing the defensive and conservative instincts of the Scottish electorate. No doubt last week’s news that British output has begun to recover after the worst recession in living memory was greeted with the same sense of relief in Bute House as it was at the Treasury.

Combined with the continued narrowing of Labour’s Westminster poll lead and an increase in English anti-European sentiment, a period of sustained growth (however modest) could help swing things in Salmond’s favour over the coming 13 months. The launch in November of the SNP’s heavily trailed White Paper on Independence might have a similar effect, particularly if it succeeds in restoring the party’s credibility on a range of key policy issues, not least the currency.

Some senior nationalists think their prospects have already begun to improve. They are convinced Better Together, the official vehicle of unionism, has made a strategic error in trying to flood the media with - as the first minister puts it - “a diet of unremitting negativity”. There could be some truth to this. Even Downing Street was embarrassed by the MoD’s ludicrous suggestion that London might try to designate Faslane nuclear base sovereign UK territory if Scotland becomes independent.

Better Together’s reliance on casual dishonesty as a campaigning technique represents another potential weakness in its approach. A few months ago, it claimed a leaked Scottish government memo contained an admission from SNP finance secretary John Swinney that monetary union would mean a Westminster veto over Scottish budgets. In reality, the document did little more than acknowledge some form of fiscal agreement would be necessary to anchor any prospective post-UK “sterlingzone”. Shortly after, a Better Together press release alleged, quite baselessly, that abuse aimed at unionist politicians by pro-independence activists had been co-ordinated by the SNP leadership.

As well as lowering the tone of debate, incidents such as these highlight a serious and far-reaching problem for supporters of the Union: even if a steady flow of misinformation and innuendo is enough to win the immediate referendum battle, it is insufficient as a long-term response to the challenge of nationalism. Unionism’s struggle to articulate a compelling, progressive case for Scotland’s on-going membership of the UK lends credence to SNP claims that no such case exists.

British political leaders do not seem overly concerned with the absence of positive arguments in favour of the current constitutional set-up. They should be. Much rests on the nature of the referendum result. Assuming Better Together prevails (still the most likely outcome at this stage, despite the SNP‘s renewed optimism), failure to secure more than 40 per cent of the vote would be hugely demoralising for the independence movement, while a 40 to 45 per cent vote share could be passed off as a respectable defeat. Anything above 45 per cent, on the other hand, would ensure Scotland’s future constitutional development remained under nationalist control.

With the backing of almost half Scotland’s voters, unionists would no longer be able to dismiss independence as the obsession of a bullying minority at odds with mainstream Scottish opinion. Moreover, the 2016 Holyrood elections would become a bidding war between the SNP and the unionist parties over enhanced powers, something the unionist parties couldn’t possibly hope to win. Any subsequent increase in the competence of the Scottish Parliament would be met with growing calls from the Tory right to restrict the ability of Scottish MPs to vote on English-only matters.

All this underlines the need for Better Together to do more than simply point out the contradictions and inadequacies in the SNP’s independence proposals. Unionism’s intellectual credibility depends on a clear explanation of how Scotland’s social and economic life will benefit from London government over the next 10 or 15 years. Currently, unionists are ahead in the polls but losing the argument. Stemming the tide in 2014 is one thing; holding it back indefinitely is quite another.