Imagine if big-name brands marketed themselves the way political parties run campaigns. Fast-food chains would denounce each other as pedlars of fatty poison. Car manufacturers would publish dossiers showing how other vehicles were prone to crash. Supermarkets would arrange for economists to sign letters to the FT claiming that shopping elsewhere was a guarantee of personal ruin.

There are regulations to stop competing companies fighting too dirty, but there is another unwritten constraint: a shared interest in not trashing the industry as a whole. If McDonald’s and Burger King ran around calling each other’s products junk, the combined message would be anti-burger. The result: fewer customers all round.

Politics has yet to reach that insight. Of course, much more is at stake in a general election than a high street purchase, and reducing parties to consumer-style brands traduces their rich histories and intellectual traditions. But shrinking numbers of voters are moved by those venerable pasts. Many see David Cameron and Ed Miliband as items from the same “politician” range, of which “Tory” and “Labour” indicate marginal differences of flavour.

So when Conservatives insist that Labour would bankrupt the nation and Labour says the Tories plan to scrap the NHS, the effect is a blended signal about malevolent politics. Everyone denies they are running negative campaigns. It’s an irregular verb: I set out the choice facing the country; you have nothing positive to offer; he/she/it drags politics into the gutter. But a mutually destructive pattern has emerged, with the main parties unable to achieve definition other than by mongering fear of each other, which makes them seem more alike in tone.

Even when leaders aspire to break free of the negativity, reactionary tendencies in their parties hold them back. Cameron’s enthusiasm for coalition was knocked out of him by the rebellious rump in his party. When Miliband experimented with a less combative style in the Commons, the heckling fraternity on the Labour benches lodged a complaint.

The likelihood of another hung parliament has added new frenzy to the business of collective political brand vandalism. Labour desperately needs to stir up the rage of former Liberal Democrats who saw partnership with the Tories as treason. Cameron is desperate to pin an imaginary alliance with Scottish nationalists on to Miliband, describing the prospect of a deal with enemies of the union as despicable. As a result, Labour has ruled out a formal coalition with the SNP ministers while carefully preserving the option of some looser arrangement that might ease Miliband’s path to Downing Street in the absence of a Labour majority.

In reality, Cameron is the leader whose tactical interests are most aligned with Nicola Sturgeon’s. The damage that the SNP threatens to inflict on Labour boosts the prime minister’s chances of clinging on to his job far more than any lavishly funded Tory campaign. The crowning irony is that this electoral windfall is a perverse consequence of his party’s brand toxicity north of the border. The image was so bad that the pro-independence movement could campaign on pledges of emancipation from the “Tory” English. By sharing the pro-union platform with Cameron, Labour MPs found themselves tarnished as Westminster establishment collaborators: “Red Tories”, as the Nats now say.

Disaffection with Scottish Labour had a longer gestation, but it was the background of a Tory-baiting political culture that lulled the party into complacency. In private, Conservative MPs cannot help indulging in a spot of schadenfreude. As they see it, Miliband’s party is reaping the harvest of decades spent portraying the Tories as alien and hostile to Scotland. Labour let the anti-politics cauldron simmer, never expecting its contents to be tipped over their own heads.

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Meanwhile, from Sturgeon’s point of view, having Cameron continue in Downing Street would confirm a trend of political systems diverging. The logic of independence is reinforced when the prime minister’s party was backed by fewer than one in five Scots.

So the Tories are gaming the anti-Westminster mood in Scotland for short-term advantage and dressing it up as concern for the union. This fits into the wider strategy of dividing the possible election outcomes into two simple categories: “competent” Tory majority rule and multi-party “chaos” under Miliband.

A variation of this argument was made in final week of the 2010 election campaign. The Tories turned their guns on the fictional “hung parliament party” with a sinister agenda to paralyse Britain. It didn’t work, and the threat turned out to be bogus.

The truth is that functional government that crosses party lines is perfectly feasible

Of all people, Cameron has the least cause to complain about coalition. It has given him five years as prime minister. It has allowed him to enact huge chunks of Conservative policy, along with Lib Dem measures such as raising the income tax threshold, for which he now tries to steal credit. It removed his parliamentary reliance on backbenchers who hate him. It gave him, in Nick Clegg, a deputy who became the lightning rod of public contempt when the advertised new politics started to look too much like the old kind.

A Conservative majority looks less likely now than it did in 2010, and Cameron knows that his passport back to power is probably a deal with a surviving band of Lib Dem MPs. Some of Clegg’s closest allies believe the Tory leadership is calibrating its position with that outcome in mind. I have heard it whispered on both sides of the coalition that the Tories are “soft-pedalling” in seats where Labour might gain at Lib Dem expense, including Clegg’s own Sheffield constituency. Elsewhere, of course, normal hostilities are maintained.

The truth is that functional government that crosses party lines is perfectly feasible. It is an established feature of local councils and devolved administrations. But Labour and the Tories treat that as a dirty secret never to be aired in a campaign. They believe in the purity of their own brands as the good kind of politics, vulnerable to contamination by others. It is a delusion sustained by tribal supporters, whose numbers are in decline.

Pandering to that tendency, insisting that fixing politics is a job made for one party alone, is part of what drives people away. It is a force dragging the whole Westminster business down.