IT'S customary after TV leaders' debates for BBC correspondents to say "there was no killer blow and no absolute winner".

But in Thursday's mega debate there clearly was one, and the killer was Nicola Sturgeon.

The post-debate instant polls were all over the place, but all showed that the First Minister was more than a match for the UK party leaders in terms of voter appeal. And that's no small achievement for the leader of a party that, remember, wants to leave the UK.

Ms Sturgeon was appearing before a UK TV audience which had been schooled by the more rabid sections of the press to believe that the SNP were simply nationalist troublemakers. At the very least, they have been regarded with suspicion as a destructive expression of "anti-politics", rather like Ukip.

Well, that casual equation of Ukip and the SNP - still common among the metropolitan Left - simply won't work any more. Sturgeon made that clear by slapping down Nigel Farage for his rebarbative remarks about immigrants and HIV.

How ironic that it was left to a Scottish Nationalist politician to make the positive case for immigrants as net contributors to the UK economy.

Nor will the UK parties be able any longer to suggest that Alex Salmond is still somehow the real leader of the SNP. It is a tribute to the Scottish National Party to have thrown up two top-class leaders in quick succession.

But it's clear that Alex Salmond's decision to hand over power to Nicola Surgeon on referendum night was the best thing he could have done for the party. Nothing became Alex Salmond in politics quite so well as the leaving of it.

Sturgeon is no token woman, still less a stand-in. She is establishing herself as one of the most powerful voices on the British left, able to articulate a coherent social democratic vision on education, immigration, the NHS and austerity. Social media has been alive with English Labour supporters asking if they can emigrate to Scotland.

Gender here is irrelevant. As was the rather patronising praise for "the women" (didn't they do well). She is not a woman in politics; she is simply one of the most impressive politicians around at the moment. Sturgeon has that rare combination of conviction and clarity of mind, an ability to speak intelligently in a language anyone can understand.

It's true that the smaller parties have a certain advantage in these debates. They have novelty, and because they're unlikely to form the government they can speak their minds without having to answer for the consequences.

But the difference with Sturgeon is that she already is in government, as First Minister of Scotland, and does have to answer for what she says. Not for her the "luxury of opposition". Unlike Natalie Bennett and Leanne Wood, who both did well in the debate, she is the real thing: in office and in power.

Sturgeon had just come from a rather bruising encounter at First Minister's Questions with another women in leadership, Labour's Kezia Dugdale, who had teased her over the Scottish Government's equivocation on full fiscal autonomy.

The SNP is refusing Labour's challenge to amend the forthcoming home rule bill in such a way as to abolish the Barnett Formula and finance Holyrood solely with Scottish taxation revenue.

Ed Miliband used this to cast doubt on Sturgeon's anti-austerity credentials and the credibility of her own spending projections. But on the night it didn't stick. Fiscal autonomy is a complex issue that few even of its own advocates really understand.

What was clear, though, was that Sturgeon and Miliband were speaking much the same language. She even said "I'm backing Ed" on raising the top rate of income tax - a conscious echo perhaps of the "I agree with Nick", which became the catchphrase of the Tory/Liberal Democrat coalition.

Jim Murphy wasn't present but he was the biggest loser of the night, and he must be fuming. Just when he had succeeded in rattling the SNP over the dodgy arithmetic of fiscal freedom, Sturgeon wipes it off the map by becoming the talk of the UK General Election.

Labour in Scotland desperately needs to make the SNP look like - in his words - "the Tories' little helpers". But the attempt to equate the SNP with the Tories was always untenable, and now it simply looks daft.

Ed Miliband did not repeat the canard that "the biggest party gets to be the government", which is still being recycled on Labour's Scottish election literature. He knows perfectly well - as now do the voters of the United Kingdom - that we are in a different world of minority politics now. The old two-party assumptions no longer work and the proof of it was this seven-handed debate.

You could almost see Ed Miliband's mind working on how he could sell a pact with Nicola Sturgeon, now that she seems to be popular in England as well as in Scotland. There cannot be a formal coalition, of course. Neither side wants that. But how about a minority Labour administration supported issue by issue by the SNP?

He could say to the country: "Look, I am not allying with a party that wants to break up the UK. But this is a democracy. Every MP has the right to vote. Can I help it if SNP MPs vote with me on Europe, raising the minimum wage, restoring the 50p tax band?" And, er, keeping him in power when it comes to the crunch.

Ed Miliband has also improved his General Election standing as a result of his television outings. Slowly but surely he is leaving the geek image behind him. He still seems over-rehearsed but his "hell, yes" style is catching on. He behaves like someone who knows they look a little odd, but, hey, who isn't? He's well up for it.

This was a fascinating week in a General Election campaign that's turning into a real corker. Politics is being changed before our eyes. Things are possible that seemed outlandish only a few weeks ago. Who would ever have guessed that an SNP leader could have topped the YouGov approval charts after a UK political debate dominated by three women leaders?

David Cameron looked uncomfortable, as well he might. He knows that the arithmetic is against him and that his days as PM may be numbered. Nick Clegg ended the coalition in forceful terms on Thursday and opened a line to Labour on taxation. The bookies will now be taking odds on a Labour-Liberal Democrat coalition with tacit SNP support.

Nigel Farage won the anti-immigration vote by revealing himself to be even nastier than his own reputation. But it is a good thing that this strand of politics now has a national voice, if only so that it can be challenged.

Last week we saw an image of a diverse political future that can only be good for democracy. No-one can say voting doesn't change anything.

And at the centre of the action is a Scottish Nationalist politician who never wanted to be there in the first place. But there you go; politics is full of paradox. And who cares? A star is born.