Was there anything offensive that Boris Johnson didn't call Nicola Sturgeon and the SNP yesterday?

"Scorpion", "Lady Macbeth", "King Herod" and "voracious weevil"; I'm only surprised he didn't call her a cockroach but I suppose the Sun columnist Katie Hopkins has the copyright on that term, which she used to describe migrants seeking to cross the Mediterranean.

Mr Johnson often goes over the top and, of course, he pretends it's all some kind of jest. But the London mayor is being seen as a future Tory leader. He knows how his words are being received in this increasingly acrimonious General Election campaign. Imagine if an SNP politician had said that about him?

Using this kind of language is not only offensive, as Ms Sturgeon pointed out yesterday, it is also counter-productive. The demonisation of Ms Sturgeon just enhances her already buoyant popularity. Are the Tories determined to do the SNP's job for them?

And let us hear no more in this campaign about the hateful, abusive cybernats. They are being comprehensively out-nastied by the Unitrolls who seem to have taken over the UK press.

The DailyTelegraph blogger Iain Martin was all over Twitter yesterday calling the SNP's manifesto launch event a "Nuremberg Rally". They do this partly to elicit abusive online responses which they can then print as examples of offensive nationalism.

Increasingly, though, the SNP's legion of internet supporters are getting wise to this tactic and are using ridicule instead of hate-speak.

But again, no Scottish voter - or English voter for that matter - could have seen Ms Sturgeon, an articulate young woman, as an extremist. Her language is entirely in the tradition of British social democracy.

Not only were there no anti-English remarks; there was scarcely a mention of independence in her speech. The manifesto is explicit that the SNP do not see this General Election as an opportunity to reopen the issue. Indeed, the SNP leader seemed to be the only politician yesterday who wasn't talking about the national question.

Nicola Sturgeon has instead emerged as the most articulate and persuasive figure on the British left. She is a remarkably assured and intelligent woman offering to work constructively with Labour and other parties to promote moderate progressive policies such as the 50p tax band, the mansion tax, £8.70 minimum wage, 100,000 houses and a "fiscally responsible" Scottish Parliament. You don't have to agree with her.

But opposing renewal of Trident doesn't make her "the most dangerous woman in the world" as the former Mirror editor Piers Morgan put it yesterday.

David Cameron had set the tone on Sunday on the Andrew Marr programme when he allegedly refused to share a sofa with Ms Sturgeon.

His condemnation of the "frightening prospect" of "a group of nationalists being involved in altering the direction of our country" was not only unhistorical (do they not teach the history of Irish Home Rule at Eton?); it was also irresponsible.

His "Carlisle Principle" holding Holyrood to account was expressed in needlessly divisive terms, suggesting that Scotland seeks to damage the English NHS and the economic interests of the rest of the UK. He's doing a very good job of that on his own.

Anyway, the idea that a wealthy country of 60 million could be seriously damaged by developments in this threadbare corner of the UK is patently ridiculous. The legislation implementing the Smith reforms on devolution already contain provision for there to be "no detriment" to other parts of the UK .

Mr Cameron has been badly rattled by the polls and by Ed Miliband's emergence as a serious political challenger. His attempt to raise the nationalist bogey is an act of desperation

But he underestimates his own voters. People can see that Labour is not being "held to ransom" by Ms Sturgeon, as parts of the UK press have been saying in screaming headlines. If anything it is the other way round.

She seeks no coalition with Labour and has no bargaining position, having promised never to assist the Tories into government.

And voters aren't so stupid that they can't see Mr Cameron failing to rule out working with Ukip or the DUP MPs from Northern Ireland. This is electoral hypocrisy; worse, it's silly.

And it is also a daft way to try to save the UK by demonising MPs from one part of it and suggesting that Scotland's MP are only acceptable provided they vote the way Mr Cameron tells them to vote.

With friends like these, the Union doesn't need enemies.