Even those of us who sniffed the Liberal Democrats' distant death on the first fragrant breeze of coalition must be gagging today at the gangrenous stench. Their failure to counter the most poisonous instincts of their senior so-called partners has rushed them to the thin line between coalition and collaboration. They seem to have one foot in Vichy already.

The assault by George Osborne on the infirm, the inadequate, the melancholy, the enfeebled and those entrapped into unemployability by incapacity benefit crosses a borderline itself; or rather it speeds the march across the line separating responsibly addressing unsustainable public debt from plain vindictive wickedness.

Since the policy of bullying the vulnerable off this benefit began under Gordon Brown, the Labour Party cannot defend further victims of its own tabloid-driven, amoral cowardice now that George Osborne promised dramatically to speed the process up. You may recall how David Freud, who retired from investment banking in his early fifties when that noble calling came to bore him, went from self-proclaimed total ignorance about welfare to writing a reformist white paper in a whirlwind three weeks. Some of our great heroes had disabilities, explained the now Lord F, citing Nelson's ocular shortfall, Churchill's black dog and Stephen Hawking's motor neurone disease, and they didn't lounge around all day eating KFC and watching Oprah.

The then Work and Pensions Secretary, James Purnell – whom I suspect from a brief encounter has come to regret this – adopted the proposal, and talked tough about "people playing the system". The weird thing, given how New Labour tough-talk usually meant banking a few nice headlines from feared editors and doing nothing, is that he meant it. Over the last year, an alliance of ministers, benefits assessors and GPs has already driven many off a benefit designed not solely for the transparently sick and disabled, but for the less instantly identifiable group who are, as the name suggests, incapable.

Incapacity is a subjective term, designedly so, allowing doctors to interpret it according to the direction in which a government is bribing or blackmailing them at the time. The case of an acquaintance from a Turkish bath may give a flavour of the callous cynicism and treachery.

This guy was shoved on to incapacity benefit in the earliest days of New Labour. Troubled and depressed, for perfectly sound reasons, he, like millions of others, was forced to take it solely to reduce the unemployment statistics. The irony is that where, with a little medical support he was probably capable of working then, but was actively encouraged not to, today he certainly is not.

Consider his appeal to an employer when he applies for one of the precious few jobs available in his part of urban deprived east London, as he repeatedly has since the doctor who declared him unfit in 1997 had an epiphany and reversed his judgment. Now in his mid-40s, this man arrives for the interview, to stack Tesco shelves or wash dishes in a school kitchen, with a CV naturally tending towards the threadbare, and the first question is this: why haven't you done a day's work in a dozen years? At this point he must either explain that all those years ago he was declared incapable of work on psychiatric grounds – what a glorious tie-breaker in a crowded job market – or end the charade with a wryly resigned "I'll get me coat".

To the eyes of Lord Freud, now a minister at Work and Pensions, the daily interest on whose savings doubtless dwarves the £69 per week this man no longer receives, and to that heir to wallpaper millions, Mr Osborne, this chap would look perfectly capable of holding down a job. Perhaps he is. But with scores of school leavers chasing every menial, minimum wage post, he is as capable of finding one as if he were in a persistent vegetative state.

There are people with far more severe psychological disorders, or in chronic and debilitating physical pain, anecdotally being treated with the same degrading contempt he says he endures when he makes the nihilistically futile fortnightly trudge to the employment office. They are the easiest possible targets for a Tory-led government rushing to cash in on honeymoon goodwill, and the absence of any opposition worth the name, by saving money in ways that ingratiate it with those portions of the media who think Mr Roget should have listed "scrounging filth" under "benefit claimant". The beauty of kicking people when they're down, without a prayer of ever rising again, is that they cannot kick back.

They can be defended, however, and so they must. With Labour fatally compromised on the issue, it falls to the Liberal Democrats or to nobody. So what was Nick Clegg thinking in acquiescing in the promotion of the born-to-crush sneer of superiority on Mr Osborne's Bullingdon face to official coalition policy? Is he a Trojan Horse planted in Downing Street to facilitate the sacking of his party more quickly and irredeemably than imagined? Or does he have a signed photo of Marshal Petain on his bedroom wall?

Already equalities minister Lynne Featherstone has attacked this persecution of the walking wounded with such barely coded savagery that she cannot honourably remain in office another week, while Vince Cable's anguished appearance on Question Time a few days ago suggested a soul riven by existential crisis. Heaven help us if that lightweight opportunist Simon Hughes casts himself as General de Gaulle, but someone must challenge this vile spectre of the most privileged scapegoating the most vulnerable.

What remaining electoral appeal the Liberal Democrats have if not as resistance fighters against the brutalist instincts of their "partners", I cannot imagine. The day the coalition was formed, it was clear Mr Clegg had signed their own death warrant in their own blood, but equally obvious that the horrendous electoral maths left him no choice. Bleeding-heart Lib Dem voters like me had grave doubts but gave him the benefit on the grounds that he had no capacity to do anything else.

Already sympathy for Mr Clegg for being dealt such a rotten hand, and admiration for how cunningly he played it, is evaporating. If they vanish entirely at the next election after cravenly collaborating in a regressive abomination like this, good riddance to the bloody lot of them.