'Yes we can" is a bold, uplifting phrase, irredeemably associated with Barack Obama's 2008 presidential campaign.

At around the same time he was about to conquer the White House, the SNP was about to lose the Glenrothes Westminster by-election. I remember activists waving, rather forlornly, "Yes we can" leaflets at the count in Fife.

Late last week, meanwhile, the First Minister, making the best of a bad situation following Mr Obama's rather half-hearted endorsement of a "united" Britain, resuscitated the phrase and threw it back at the leader of the Western world.

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It was a neat trick, but, even uttered by the talented Mr Salmond, those three words sounded a little desperate. This wasn't entirely the First Minister's fault, for it's been Obama's term and a half as president that's rendered "yes we can" full of disappointment rather than hope.

Of course the success of Obama's first hope-fuelled campaign has been overstated, for at the time his team deployed ad hominem attacks on opponents' integrity while during his second - also successful - campaign for the White House, negativity (in relation to Mitt Romney) almost completely trumped the previously vaunted positivity.

Mr Obama's record in office has also served to undermine his hope agenda. By promising rather too much, the president inevitably disappointed, not just those who wanted him to fail (Republicans) but also those who earnestly wanted him to succeed. Despite some successes (albeit truncated, Obamacare remains a major achievement), the lustre came off Mr Obama long ago.

Nevertheless, the SNP (during the 2011 Holyrood campaign) and later Yes Scotland imbibed a rosy assessment of Mr Obama's rhetoric and placed it at the heart of its campaigning strategy. Yes guru Stephen Noon rarely misses an opportunity (echoed by Salmond) to argue that positive campaigning will always beat negative campaigning.

Although simplistic (a negative campaign, for example, defeated AV in 2011) it's become almost an article of faith for supporters of independence who have successfully framed September's referendum as hope versus fear, positive against negative. Both campaigns, in reality, are a mixture of the two; even Yes indulges in personal attacks on "English" politicians and warns - risibly - that a No vote risks seeing the NHS privatised and tuition fees reintroduced in Scotland.

This is not, I hasten to add, a defence of relentless negativity on the opposite side, but as I've argued before it's possible to be too positive as well as too negative. An unnamed psychologist quoted in a Sunday newspaper hit the nail on the head in observing that supporters of independence were "overdoing it". 'There is a credibility gap,' he or she added. 'They are prepared to discuss only the sunny, positive stuff - but that it not the reality of people's lives.'

Thus, as Pete Ramand and James Foley argued in their recent book, Yes: The Radical Case For Scottish Independence, "Boosterism and hype…replaces substance and policy as the root cause of success", reducing "political communication to a dichotomy, positive and negative." Not only has that failed to turn Yes Scotland into a vote-winning machine (at least judging by opinion polls), but such a reductionist approach is arguably "indefensible in practical and intellectual terms".

Indeed, the Scottish Government's own (mixed, but broadly successful) record in office further demonstrates the dangers of emphasising aspiration over realism. This week, for example, Alex Salmond will launch a blueprint to "reindustrialise" Scotland, conveniently forgetting he promised the same during the 2011 election, only for it to be forgotten about until now.

Tomorrow, meanwhile, it seems likely Scotland will once again - as in 2010 and 2011 - fail to meet its much-hyped carbon emissions reduction target, a stark gap between the "positive" goals enshrined in the Climate Change (Scotland) Act and environmental reality. Both reindustrialisation and reduction carbon emissions are, of course, laudable aims; but difficult to deliver.

In other respects the SNP has become trapped by its own logic, ie it is relentlessly upbeat about the Scottish economy while attacking the economic mismanagement of Westminster; perennially hopeful of agreeing a currency union having, in the past, dissed the Bank of England as too focused on the south-east and the pound as a millstone.

Last week, welfare policy also fell victim to what the aforementioned psychologist called a credibility gap. Responding to the second report of the Scottish Government's Expert Working Group on Welfare, John Downie of the Scottish Council for Voluntary Organisations (not exactly a frequent critic) said he admired its positive approach but noted its principles weren't reflected in the recommendations which, he added, "could easily just result in a slightly more compassionate version of the current system".

A slightly more compassionate version of the status quo would at least be a realistic aim; indeed despite repeatedly gloomy (and therefore "negative") economic forecasts from the Institute for Fiscal Studies (to which a "positive" response usually sounds unconvincing) the best-case scenario for an independent Scotland would be one of economic stability, perhaps with slightly higher growth and a modestly more generous system of welfare.

For many that would be enough to justify a Yes vote, but it's still far removed from the sunlit uplands frequently posited by the prophets of positivity. Few seriously doubt that Scotland could sustain itself as an independent country, but that isn't the same as arguing it'll be the fourteenth (previously sixth) richest country in the world and somehow able to duck the economic challenges faced by most other developed countries. Instead relentless positivity simply fuels the fantasy politics of both left and right, where independence will facilitate a socialist utopia or tax cuts will lead to unparalleled economic growth.

The trouble is that beyond a small band of diehards on either side of the ideological spectrum (some, quixotically, attempt to reconcile the two) few instinctively find either scenario credible and thus such arguments are subject to the law of diminishing returns. The more that's promised the less credible it appears.

So with 100 days to go (actually 101, but neither side appears to mind), where do things stand? I reckon the negative component of the Yes campaign is steadily increasing. Blair Jenkins recently estimated it stood at 20 per cent, which, interestingly, means it's already breached Stephen Noon's "no mixed message" rule. By contrast, the positive element of Better Together looks to be on the rise, albeit modestly.

Last week it took ownership of the word No, but casting it in more positive terms such as "I'm voting NO" and "NO thanks". There's also a discernible increase in confidence. At a Better Together meeting at Leith Academy last Thursday I was struck that the constitutional lawyer Adam Tomkins felt able to declare he was "damn proud" to be British, and greeted with an enthusiastic round of applause.

Positivity has its place in modern political campaigning, but it has to be tempered with realism, something the US President now understands only too well. He wrote eloquently about the "audacity of hope", and while that took him across the finishing line, political reality hasn't lived up to the hype.