SO Labour's Scottish agony intensifies. The party which is delivering its widely-praised devolution settlement in double-quick time now looks on, with a mixture of bewilderment and hurt, as the SNP surges past it in voting intentions for the Parliament Labour is creating with such all-consuming energy at Holyrood.

With his well-known fondness for historical parallels, Donald Dewar's private thoughts might just be turning to how Churchill must have felt in 1945.

In three months, without the nationalists saying anything of any real significance about what they would do with the powers about to be transferred to Edinburgh, an 11-point Labour lead in voting intentions for the new Scottish Parliament has been turned into a five-point SNP advantage in The Herald's monthly System Three surveys.

Now that a trend is clearly emerging, the big question is why. Why, with exactly a year to go to the first elections, do more and more Scots seem prepared to vote Nationalist for a subsidiary Edinburgh Parliament, than did last May - or would do now - in elections to the ''sovereign'' Westminster Parliament?

If this is another surge in Scottish support for full independence - the appearance of that legendary slippery slope at the foot of Edinburgh's Royal Mile before a single brick has been laid on Mr Dewar's favoured site for the new Parliament - why is that surge not equally apparent in voting intentions for Westminster?

The polling facts are simply stated. Since we first asked System Three to pose separate questions about voting intentions for the Westminster and Edinburgh parliaments back in February, it has been clear that significant numbers of Scots see nothing irrational in voting Labour at Westminster elections and SNP for the Home Rule Parliament in Edinburgh.

Since evidence of that differential voting behaviour was first identified, Labour has gone on losing support - and the SNP has gone on gaining it - for both the Scottish and Westminster elections.

But Labour's haemorrhage has been heavier in voting intentions for the Edinburgh legislature, accentuating the differential that was already there when The Herald first asked the question.

Between February and May, System Three puts the swing from Labour to the SNP in Westminster voting intentions at 5.5%. But the equivalent swing for the Scottish Parliament elections is now a full 8%, enough to put the SNP in a clear 5% lead over Labour although it still lags by 14% in voting intentions for Westminster.

Why? I doubt anyone, including Alex Salmond and the rest of the SNP leadership, really knows why. Differential voting behaviour is nothing new. And, as another poll for Scotland on Sunday last weekend showed, some Scots voters seem determined to create a second-order differential by casting their first-past-the-post and regional list votes for different parties in next year's inaugural Home Rule elections.

When we get down to the reasons why voters behave in these increasingly complex ways, there are few easy off-the-shelf answers.

Clearly, despite its heavily-endorsed Home Rule settlement for Scotland, New Labour has endured some very tricky times north of the border since it was elected so dramatically last May.

The creation of a Scottish Parliament presents Scottish voters with a much more effective way of registering their concerns about wayward MPs and municipal cronyism than was ever available under the unitary Westminster model.

Then there is the vexed question of whether New Labour plays as well in Scotland as it manifestly continues to do across great swathes of Middle England. Bound up in that debate is the more specific one about Tony Blair's personal popularity north and south of the border. Conventional wisdom would have it that Scotland, with its more collectivist and redistributist instincts, and an enduring pride in what investment in a healthy civic realm can achieve, harbours enduring suspicions about New Labour in general and the Prime Minister in particular.

Whether that oft-repeated wisdom is accurate is not entirely clear. Yet, despite Scottish antecedents and an Edinburgh schooling, Tony Blair's own body language whenever he ventures north always seems to underline some feeling of mutual unease.

The fact that sources around Downing Street regularly give vent to their feelings about ''ungrateful Jocks'' simply adds fuel to the blaze.

The suspicion grows that, if all but a handful of Scottish Labour MPs see their political futures at Westminster, then the party that is delivering on its constitutional pledge doesn't, in its heart-of-hearts, regard the new Holyrood Parliament of supreme importance.

And if that's their attitude, more Scots voters may be deciding: why not vote for a party whose very name suggests it has Scotland's interests at heart and whose Westminster MPs are all itching to get back to Edinburgh?

The SNP faithful will doubtless be in triumphalist mode this morning. But Nationalists cannot simply assume that these poll findings are all simply down to more and more Scots becoming relaxed at the prospect of full independence.

The SNP will pay a heavy price if it misjudges what these polls are really telling us.

I doubt whether all of the growing numbers of Scots voters willing to vote SNP next May want the Nationalists to use the new Parliament as a battering ram for independence.

Many, I suspect, simply want Alex Salmond and his team to run the Holyrood legislature better than they judge New Labour would. Real Nationalists, in their impatience for the next stage, may find that more modest expectation hard to fulfil.