Nothing spooks the middle classes like a perceived assault on the value of their homes

House owners are nervously calculating the effect that a Labour mansion tax would have

A Regent’s Park mansion on Chester Gate, which is one of London’s most expensive properties (Rex)

As if Ed Miliband didn’t have enough to contend with... While Ukip begins to make inroads to the working-class Labour vote, the party’s leader should watch out for the wealthy left-leaning middle class, whose support will vanish like water in the Sahara if a forthcoming electoral manifesto threatens to introduce the thing they most fear, namely the mansion tax.

Should a future Labour government, (or indeed a Lib/Lab coalition) impose a punitive tax on all properties worth more than £2m, all those people living in the traditional lefty London heartlands of Hampstead, Islington (my own manor), and yes, even Hackney (where house prices are shooting up) will run for the hills. They simply won’t vote for him.

Having sat cosily in their/our houses for a decade or so, gleefully watching them crawl up the price barometer from half a million to one, two, three or four million without so much as lifting a paint brush, house owners in central London, and other parts of the country (the Cotswolds, Cheshire) are now nervously tapping their postcodes into Zoopla in order to work out just how big the bill could be should Miliband obtain the keys to No 10. It’s like a nightmarish Monopoly scenario where everyone suddenly lands on Mayfair. With a hotel. Not owned by you.

Ed Balls, whose own north London home almost certainly surpasses the £2m bar, suggests that the party could raise up to £2bn from the tax. Because our houses are all now worth so much. The tax would only kick in for values above the £2m threshold (so if a house is worth £2.5m, the householder would pay tax on the £500,000), but prices are now so giddy, particularly in London, that this would still mean bills arriving on thousands of doormats in the capital. Annually. People are going to be cross. Very cross.

The tax would take zero account of income or, indeed wealth, outside of the (notional) value of one’s family home. Bad for the middles; very gloomy indeed for those who live in valuable Victorian piles bought decades ago but who are no longer earning, have next to no interest on their savings, and are living on a basic pension. Miliband’s traditional London fanbase might start to take a dim view of his policies once they have to start bailing out their parents.

Of course, the threat of the mansion tax from the Opposition is probably the only silver lining in the Prime Minister’s current cloudy forecast. An investigation by yesterday’s Independent on Sunday discovered so much anxiety about the mansion tax threat that high-end property developers have started to funnel funds to the Conservative party.

But surely the mansion tax is sort of Robin Hoodish, and as such A Good Thing? Surely wealthy people who are lucky enough to live in lovely houses – or indeed flats – worth a fortune won’t mind spreading the joy a little bit? Are you kidding! First, everyone will hope their houses go into reverse gear, and start devaluing like crazy. Well, if that happens, isn’t that a rather excellent outcome? After all, aren’t we very concerned about younger people being priced out of the market, about nurses and teachers and the lower-paid not being able to afford anything other than a cupboard to live in? Well, we might be concerned, but compared to the ignominy of having our house collapse in value, such concerns will vanish in the dust.

Publicly everyone criticises house values soaring; privately, it’s a different matter altogether. House worth is so crucial to the British mindset that a deflation of values would be seen as a personal mark of shame. No matter where you are on the political spectrum.

The head of London property company Glentree International, Trevor Abrahamsohn, told the Independent on Sunday that if Labour wins in 2015 and introduces a mansion tax “there will be a collapse in the property market which will start at the top and cascade down. And before you know it you will have a property market which is in recession by self-engineered means. That is the biggest own goal that any country could commit.” Last year Abrahamsohn’s company gave the Conservative Party £12,650 and he has said that because of the mansion tax threat, he will continue to give more.

As with Abrahamsohn, so with the chattering class. The squirming about the mansion tax will go right through it, affecting all bar the noble Balls. It is just the latest example of how difficult it is to square social necessity with personal gain. Car ownership, catching planes: bad things unless you are personally affected. In which case they are “essentials”.

It’s like swimming pools in LA; people with pools know they are selfish and that it is wasteful to have a turquoise oasis in your back garden. But get rid of your pool? You first, brother. Austrian artist Alfredo Barsuglia has made an art work out of it, marching into the Mojave Desert and building a swimming pool there to draw attention to the madness of California. “That is one problem with the capitalistic democracy, Barsuglia says. “Nobody takes personal responsibility for society.” Indeed.