After independence defeat, SNP looks set for general election victory in 2015

Forty-five is a losing score in a referendum, but as a percentage in a general election, it would provide a crushing victory.

That is, in essence, why the Scottish nationalists look increasingly likely to move from the despondency of defeat in September to an historic win next May: they may not be a majoritarian party, but they currently seem to be reliably speaking for close to 45% of Scots.

All told, the Scottish National party’s 43% projected share in a poll of Westminster voting intentions north of the border for the Guardian is only a whisker below the 44.7% who opted for independence. Their current Westminster tally is strikingly close, too, to the 45% of the constituency vote that gave Alex Salmond his great Holyrood landslide in 2011, and indeed to the 44% who tell ICM in Friday’s survey that they would plump for the nationalists if there were a fresh ballot for their local Holyrood seat.

In the 2005 and 2010 general elections alike, the SNP performed poorly at Westminster but nonetheless went on to come out ahead at Holyrood – by a single seat in 2007 and then by a landslide in 2011. “Scots then looked like making a habit of split-ticket voting,” explains Martin Boon, of ICM Unlimited, “favouring a strong nationalist presence in Edinburgh, while continuing to regard Westminster elections principally as a choice between the UK-wide parties. Perhaps the most important finding in today’s poll, however, is that the old distinction between UK and Scottish elections has ceased to apply.”

There is little doubt that it is the independence referendum that has catalysed this change. While Greens and other radicals swelled the ranks of the separatists in the Autumn, it was former Labour voters who did most to boost the SNP loyalists and secure the cities of Dundee and Glasgow for the pro-independence vote.

Now, these voters appear almost entirely lost to Labour – Ed Miliband retains a mere one voter in every four of those who backed Gordon Brown in 2010 but then went on to side with yes last September. The SNP by contrast, is currently mopping up just over two-thirds (68%) of these “Labour yesses”, as well as 81% of yes voters overall.

The results in September suggested that Labour yesses were concentrated in the heartlands, and Friday’s poll fits with that. The ICM survey establishes that this group of former party supporters, which will, one way or another, prove fateful for Labour next May, is increasingly emerging as a cohesive bloc.

They overwhelmingly regard themselves as Scottish rather than British – 80% when forced to choose between the two identities, compared to 58% across the population as a whole. Some 63% said they are convinced that the all-party Smith commission’s plans for extra devolution do not go far enough, compared to just 30% of the population as a whole.

A crushing majority of 77% of this group are convinced, against Labour policy, that Scotland should set its own corporation tax rates. On Trident, too, opinion is similarly emphatic: where 43% of Scots as a whole want Trident scrapped, among Labour separatists it is 72%, against only 14% who want to retain it.

It is tricky for Scottish Labour to compete with the SNP on any of these questions, even if it wanted to, since Smith, Trident and corporation tax are all areas on which there is effectively a UK-wide Labour party position.

While the first minister, Nicola Sturgeon, seems inclined to march the SNP to the left and into the industrial heartlands, what complicates things further for Labour is anxiety about losing more centrist supporters, such as former Lib Dems and even those who might be tempted by Ruth Davidson’s Scottish Conservatives. On non-national questions, the poll does not indicate any great lurch to the left among Scots as a whole.

While 26% of the overall electorate says Scotland should go for higher taxes to support better services, and only 13% would prefer tax and spending cuts, a plurality of 41% think the current balance between tax and spending is about right – suggesting that once the thirst for home rule is satisfied, the big fiscal choices made by Scotland might not be all that different from those south of the border.