THE Yes campaign cannot rely on simply securing the majority of 'don't knows' to claim victory on September 18 but must win over a proportion of those inclined to vote No, according to Scotland's leading psephologist.

The view of John Curtice, professor of politics at Strathclyde University, came as Labour's Douglas Alexander claimed time was now running out for the Nationalists to find the game-changer they sought and that their campaign was unravelling as it came up against "the granite-like resistance of Scots to embracing separation".

Mr Curtice explained that recent polls suggested the undecided were beginning to make up their minds but, as yet, the move had not significantly changed the balance of opinion in Scotland.

"In any case," he explained, "this has never been a referendum campaign where, certainly the Yes side, who have always been behind in the opinion polls, are going to win simply by winning over the undecided.

"I'm not sure there's ever been enough people who were wholly undecided, as opposed to having an inclination maybe to vote No or to vote Yes but perhaps might be willing to change their minds, for the Yes side to be able to win this referendum. They've always been having to try to reach into those folk whose inclination was to vote No but who maybe might be capable of being persuaded to change their minds."

In support of his point, Mr Curtice highlighted the average of the last six polls, which showed 43 per cent for Yes and 57 per cent for No. But he also pointed to surveys from the weekend, which gave better figures for the pro-independence campaign, suggesting the first head-to-head TV debate had not been as bad for the Yes camp as initially thought and raised the question of whether the Yes campaign had gained new momentum.

"[As regards] the psychology of the campaign, where a week ago there was some suggestion that the Yes side were at risk of being written off and the bottom had fallen out of their campaign in the wake of what was undoubtedly a rather disappointing performance by Alex Salmond in the leader's debate, it seems to be back in the game so that we can expect it to fight all the way through to September 18 - though we should remember that every single opinion poll puts the No side ahead," he added.

Meantime, writing for left-of-centre website ProgressOnline, Mr Alexander insisted the No camp's slogan of being stronger together and weaker apart had "stood the test of time", while the Yes camp was "crumbling".

He added: "With a month to go, Scots are calling time on the Nationalists's half-baked double-speak. A campaign that started by branding their opponents as scaremongershas ended up as the purveyors of threats and doom-laden predictions that owe more to desperation than to reality."