JIM Murphy was reflecting on the opening shots in the General Election battle. Speaking at a press lunch in Edinburgh, he declared that Alex Salmond had made Nicola Sturgeon's first major strategic error. The Scottish Labour leader was referring to the SNP's decision to put devo max at the heart of its election campaign. A week earlier, Mr Salmond had said the SNP would demand full fiscal autonomy as the price for propping up a minority Labour government. To the surprise of some of the Nationalists' opponents, Ms Sturgeon endorsed the remarks.

Since then, Labour, the Conservatives and the Lib Dems have struggled to contain their glee. Under a devo max set-up, Scotland would raise all its own taxes to fund public services, finance debts and contribute towards the few shared UK functions that remained, principally the armed forces and overseas embassies. An oil price of $50 per barrel would blow a multi-billion pound black hole in Scotland's public finances. Devo max would be an economic catastrophe, cried Holyrood's main opposition parties.

Each found its own way of making the point during First Minister's Questions on Thursday and they have no intention of letting up before May 7. But has the SNP really scored an own goal? Is devo max the huge blunder Mr Murphy believes it is?

SNP strategists believe Yes voters will approach the election in a very different way from No supporters. For No supporters, the referendum was a mission accomplished. Most will move on and weigh up the General Election as a conventional battle between the parties.

For Yes voters, however, the referendum, still painfully fresh in the memory, is regarded as unfinished business. They are more likely to cast their vote in a way that will advance "the Yes cause," as one senior SNP strategist put it.

The SNP's election offer has been put together with that in mind. Demanding devo max will not frighten those who voted in favour of independence, whatever the price of oil. Indeed, the more the pro-UK parties attack it as "independence by the back door," the more the 45-ers will warm to it.

Between now and May 7 the Nationalists will also campaign hard against austerity and against a new generation of nuclear weapons, two issues which united Yes voters last September.

At First Minister's Questions, Ms Sturgeon parried attacks on devo max by talking about austerity, accusing Labour of backing Conservative plans to cut an extra £30billion of public spending over the lifetime of the next parliament in a Commons vote on Tuesday. It is a charge Labour rejects and with plenty of justification (there is a vast, £25billion gulf between Ed Miliband's and David Cameron's planned annual spending cuts, as the IFS think tank noted this week). But, for the SNP, the two parties are indistinguishable austerity junkies and, however cynical, that message could well resonate with those who were persuaded last year that Labour and the Tories are peas in a pod.

SNP insiders insist their election offer is broad-based and designed to appeal to No voters who like the idea of further devolution. They cite polls showing majority backing for devo max.

Well, no-one wants to sound unambitious but the Nationalists' pitch actually looks more like a core vote strategy, albeit one aimed at the large core who voted Yes in the referendum. Never mind No voters, if the SNP can harness that level of support, it can look forward to a very good election. Averaging out this week's YouGov polls, the party yesterday said its support stood at 44 per cent, 17 points ahead of Labour. On that showing the SNP would win 48 of Scotland's 59 seats, according to one widely-used online election calculator. No-one is seriously predicting that many but the party is aiming to emerge with more seats than Labour.

The SNP won 45 per cent of the vote in the 2011 Holyrood election and the Yes campaign took 45 per cent in the referendum. If Nicola Sturgeon can now achieve the same in a Westminster election, with a clear focus on the constitution, it will be a major breakthrough. Though miles ahead in the polls, the Nationalists cannot take it for granted. Among other things, turn-out - 64 per cent in Scotland at the last General Election - will be critical. But their strategy of re-running the referendum makes more sense than their opponents are willing to admit.

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