What’s the key Conservative campaign message? Long-term economic plan? Hard-working people? Securing a better future?  No: it’s none of the above. The slogan that’s increasingly at the vanguard of David Cameron’s big push is “competence v chaos”.  And its real meaning is signposted by William Hague in the Sun on Sunday today (£).

As the Leader of the Commons demonstrates, Tory high command is not so much trying to frame the election between Strong Cameron V Weak Miliband as…Strong Cameron v Weak Miliband and Strong Salmond.

As Hague puts it: [Miliband] is “grasping at straws but has found a willing wee helper to keep his dream alive”.  (Note the mocking use of that second adjective, crafted for the consumption of English voters.) “Enter Alex Salmond, who wants to propel Labour to power to help his dream of breaking up Britain…Ed Miliband is so weak he would sell this country’s soul if the voters let him. Just imagine what else he would surrender.”

Higher taxes, more debt, the junking of Britain’s nuclear umbrella…the Commons leader goes on to sketch the nightmare product of a “so-called Progressive Rainbow Alliance of Labour, the SNP, Plaid Cymru, Sinn Fein and the Greens”. Part of this is absurd (Sinn Fein don’t come to Westminster).  More of it is convincing: such a coalition could happen.  And a big slice is certainly alarming.  Here, then, is that key message – or, rather, Key message.

For Conservative strategists point to the video above as inspiring and summarising the point they want to project.  It comes from New Zealand’s recent election campaign.  On the one hand, a sleek racing boat commanded by a united crew powers ahead – representing John Key’s National Party.  On the other, a disorderly rabble flaps around in a stationary tub – the different colours of its crew representing the opposition parties, bound together only by a common love of higher taxes.

Hence Cameron’s warning at a rally yesterday that “you could end up with an alliance between the people who want to bankrupt Britain and the people who want to break up Britain.”  Senior Tories say that voters have spontaneously been raising this fear themselves in focus groups.

By hammering home the competence v chaos case, the Prime Minister hopes to get a few more habitual non-voters out, soften Labour’s vote further, peel a bit of support off the Liberal Democrats…and, above all, snatch former Conservative voters back from UKIP.  Salmond is coming! Beware, beware! His flashing eyes, his floating hair…