TO her followers, and to a growing band of Scots who don't share her politics but admire her style, Nicola Sturgeon is the cat's whiskers.

In this election she is certainly the cat who's got the cream.

Whether Labour or the Conservatives achieve a narrow victory on May 7, the SNP cannot lose.

Ms Sturgeon yesterday stepped up her courtship of Labour, promising to support Ed Miliband's party and make it "bolder and better" in government if the parliamentary arithmetic propels him towards Downing Street.

Her "number one priority," she said, would be to shift Labour's economic plan - to eliminate the current budget deficit by 2020 - closer to her own programme of modest spending increases. In return, Ms Sturgeon promised, she would be a constructive ally at Westminster.

As the IFS think tank has pointed out, the two parties' economic strategies are not that far apart. As if to emphasise the do-ability of a loose, unwritten deal, Ms Sturgeon endorsed Labour's tax plans in their entirety and threw in support for ending zero hours contracts for good measure.

We'll not be "destructive or disruptive," she coaxed. We're not going to Westminster "to bring down governments or block budgets".

There is no reason to believe she would, despite Labour's natural suspicions. A Labour government, supported by the SNP and tweaked modestly on this issue or that, would probably go down so well in Scotland that a landslide for the Nationalists in next year's Holyrood election would almost be assured. Why rock the boat if you are helping to sail it and it's heading in the right direction?

It's one reason Labour are wary and would rather defy the SNP to vote against them than make any kind of concession. But might Mr Miliband be tempted to work informally with the SNP for the sake of a quiet life? He may trust Ms Sturgeon more than he would Mr Salmond.

This intricate footwork is not just about coming to an understanding, it's about offloading the blame if David Cameron remains Prime Minister. And that could easily happen. There is no getting round the fact that Ms Sturgeon's frequent messages to folk in the rest of the UK - completely unnecessary from the point of view of winning votes in Scotland - play directly into Mr Cameron's hands. They illustrate his posters and feed his nightmare vision of a weak Labour government dragged further to the left by a party that's not only Hell bent on wrecking the economy but the United Kingdom itself. Ms Sturgeon protested yesterday at his "huge tactical mistake" but the Tories are satisfied her every intervention wins them votes in England. Since she is also taking votes from Labour in Scotland by the barrel-load, they could not be happier with her.

SNP supporters know a Tory government would increase support for independence. Tellingly, they burst into gales of laughter yesterday when Ms Sturgeon quoted the bit in the manifesto which says independence "is not what this election is about".