PERHAPS it's the heat.

A couple of routine-looking referendum rows caught the eye this week not because of the arguments they advanced but the language they employed. On Tuesday Alex Salmond used a speech in the Isle of Man to claim that his plan for an independent Scotland to enter a currency union with the UK would be "straightforward" to negotiate. That's not the view of the UK Government, of course, but the First Minister emphasised his point with a blistering attack on the pro-UK campaign, its misleading "chronic negativity" and "catalogue of blunders".

A couple of days later, the SNP made the perfectly reasonable point that a plan to develop the Common Travel Area between the UK and Ireland was perhaps a sign of possible future co-operation with an independent Scotland.

But, again, the point was overshadowed by a furious assault on the Nationalists' opponents. SNP MSP Clare Adamson said the pro-UK campaign had "consistently argued that there would be border controls between Scotland and England if we vote Yes" (not quite true, but let's leave that aside) and added: "The basic problem for Project Fear is that it has got nothing good to say about Scotland –which is why it is reduced to tripe and dishonesty".

Unsurprisingly, these comments did little to inspire a reasoned and thoughtful debate about either the likelihood of a formal monetary union or an independent Scotland's chances of securing an opt-out from the Schengen travel area. Instead the pro-UK side accused the Nationalists of being negative. "More negativity, assertion and selective use of the facts," said Labour's new constitution spokesman Drew Smith. "Pointless name-calling" and "deeply sad," was Better Together's response.

It's unusual for such gripes to surface.

Since Alex Salmond resumed the leadership of the SNP in 2004 the party has laid exclusive claim to the language of positive politics. Senior Nationalists have consciously, publicly and, to use a favourite Salmond word, relentlessly told anyone listening they have a "positive" story to tell while their opponents are wholly "negative". So successful have they been it's become an article of faith, and not just for supporters.

It's not quite as simple as that, of course. You only have to consider one of the Yes campaign's key slogans, the endlessly repeated "Westminster isn't working for Scotland", to be reminded that the reality is somewhat different. Or think back to the last Holyrood election when a good part of the SNP's effort was devoted to personal attacks on the then-Scottish Labour leader Iain Gray.

That's not a criticism of the SNP, by the way. All political campaigns use a mixture of positive and negative messages. It's entirely legitimate – challenging and questioning your opponents' arguments is a healthy and necessary part of debate. And it's also inevitable – negative and positive messages are often opposite sides of the same coin. The claim that Westminster isn't working for Scotland, for example, is merely the flip side of Yes Scotland's positively expressed line, "Scotland's future should be in Scotland's hands". It's a quite hard to articulate one idea without the other.

What is happening, though, is that pro-UK campaigners are finally losing patience with senior Nationalists calling themselves positive in one breath then calling their opponents every name under the sun in the next. Perhaps the only surprise is how long it has taken. Labour MSPs have long grumbled privately about the SNP's successful expropriation of "positive" politics without any real response. That may be changing.

Does it matter? It's far from clear that positive, or at least positively-perceived, campaigning always trumps negative. Sophisticated US campaigns would not spend millions of dollars on blood-curdlingly negative TV ads if they were not effective. Both sides know this and the pro-UK campaign will continue to raise doubts about SNP plans just as the Nationalists will continue to argue the Westminster is holding Scotland back.

The SNP, however, will guard its carefully crafted image as the positive party jealously. It's better to be seen as upbeat and hopeful than pessimistic and doom-laden, even if the reality is slightly different. Whether the pro-UK campaign can start to dent that impression at this late stage remains to be seen.