Conservatives prey on the primal fear of those marauding Scots

Tory strategy: This is an attritional, grinding election campaign

SNP leader Nicola Sturgeon campaigning in Edinburgh on Wednesday. The Tories are focusing on the threat of an SNP-Labour coalition (Getty Images)

George Osborne has an election video on his mobile phone. It shows a rabble of misfits in a stationary boat, their oars thrashing purposelessly in the water. Labour spending commitments – and those of other opposition parties – flash up on to the screen, to the jingling sound of a cash register. But all turns out well – at least for the pumped-up team of muscular rowers who, in their sleek racing craft, power through towards the winning line.

But the attack ad isn’t from the Conservative Party. Indeed, it doesn’t hail from this country at all. It’s from New Zealand – where last year John Key’s National Party thumped the Labour opposition, fell just short of a majority, and returned to government.

None the less, it holds the key to this attritional, grinding Tory election campaign. For New Zealand’s Labour Party, substitute Ed Miliband. For its minor parties, bring on the Greens, Ukip – and especially the SNP.

David Cameron, Osborne and Lynton Crosby – the trio at the head of the Conservative campaign – are set on portraying the opposition parties as being, literally, in the same boat. But there is a British twist to the tactic that they have borrowed from Key. Or is it better described as an English one? For their gambit of portraying Ed Miliband as a puppet, whose strings are pulled at will by Nicola Sturgeon, or as a tiny prisoner in Alex Salmond’s top pocket, arguably has constitutional implications.

After all, there is a fine line between claiming that SNP MPs would dictate terms to Miliband in a hung Parliament and suggesting that they would have no right to – or that it would be better for the country were the SNP to win seats off Labour. The latter is, after all, a unionist party. Norman Tebbit, who has no love for the Prime Minister, has argued that this is a line that the Tory leadership has crossed. His fellow peer, Michael Forsyth, is concerned. Sir Malcolm Rifkind has spoken out.

All parties are fighting to get their message across to undecided voters (Getty Images)

None of this is worrying the top Conservative team in the least. And in the macho terms of campaigning cut-and-thrust, why should it? They lost the first week of the post-Easter campaign proper, in which Tony Blair’s speech on Europe and Miliband’s proposal to abolish non-dom status grabbed headlines and gave Labour momentum.

They didn’t break through during the week after, in which Labour’s manifesto proposals got a better poll reception than Tory ones.

All the while, too, Miliband’s ratings have ticked upwards. But this week’s Conservative re-singing of the weak-Miliband-strong-SNP song – led by John Major, in the Tory equivalent of Labour’s Blair initiative – has dominated the media.

Miliband’s own push on the NHS has found it hard to compete. Furthermore, the interventions of those Conservative peers and knights have actually helped to keep the story going. Cameron might as well send Tebbit a thank-you note.

But the main aim of the Tory campaign team isn’t to triumph in the Westminster Village game of Who Won The Week? It’s to persuade Ukip, Liberal Democrat and even some Labour voters to switch to the Conservatives, as the only political force that can man Hadrian’s Wall against those marauding Scots.

Senior sources claim that this primal fear was originally raised spontaneously in Conservative Party focus groups by English voters, perhaps even before the Chancellor had downloaded that video from New Zealand.

One savvy, modernising Tory Minister insists that election campaigns usually change nothing. But he insists that the SNP issue is the exception that proves the rule.

Conservative candidates in marginal seats claim that voters are switching to them, alarmed by the prospect of Salmond dictating terms at Westminster. There is precious little evidence in the polls of this to date. But the target group that may be swayed represents “a significant chunk of people”, according to Anthony Wells, of YouGov.

Will the Tory campaign change tack before polling day? Will it suddenly and convincingly project the “change, optimism and hope” that was the slogan of the early, modernising Cameron? This is very doubtful. The Conservative Manifesto saw a rush of new pledges on childcare and housing. Any further novelties would come very late in the day. Campaigns tend to revert to their key message as polling day looms into view.

Cameron, Osborne and Crosby are relying on direct email contact to do the trick, given the shortage of boots on the ground in key seats. They will stick to the plan. Their motto is the Duke of Wellington’s: “Hard pounding, gentlemen. Let’s see who pounds the longest.”

Paul Goodman is the Editor of ConservativeHome