Ask who deserves support from the state and people choose disabled children first, along with cancer sufferers. How inept to targets cuts on these, then. Members of the House of Lords, packed with experts in disability, were always likely to rebel.

An £18bn cut from the benefits spend is an unthinkable sum, bound to damage the frail – and Lord Freud's underhand procedural trick to overturn the rebel amendments will only harden the crossbenchers. Yesterday the government proclaimed no turning back, but the lords representing the likes of the disability charity Scope or Macmillan Cancer Support should make them blench.

This is classic policymaking from David Cameron. First, soften up the public: tell horror stories about the rising benefits budget. Cue benefit cheat stories of "disabled" claimants caught running marathons, or water-skiing in foreign locations. Ignore Department for Work and Pensions figures showing that disability fraud is just 0.5% – one anecdote trumps all statistics. Keep repeating Iain Duncan Smith's "the incubation of the benefits culture was one of Labour's great sins".

Who will know that numbers going on to incapacity benefits were falling in the wake of Labour's many crackdowns? Costs of disability living allowance (DLA) rise not through fraud but demographics, as the old live longer and are sicker, disabled babies survive into adulthood, an epidemic of diabetes is upon us, the mentally fragile get more help to claim benefits and challenges extend the scope of disabilities covered.

Opinion polls showed strong support for cuts, though few knew what they were – benefit details are far too labyrinthine for most news editors. To soothe alarm among decent people, Cameron said the opposite of what he was doing. His manifesto promised no cut in disability allowance, and he said "I would never do anything to hurt disabled children."

Cameron keeps forgetting that public applause is less than half the battle. Out in the real world, the wheels come off bad ideas rushed through recklessly. He will face a mudslide of awful stories: media outlets that support cuts now will soon be reporting shocking cases. Most disabled children lose £27 a week, and families with two disabled children lose twice as much. More than 40% already live below the poverty line, as these mothers can't work and have extra childcare costs. Expect heart-rending stories from the 25,000 young carers looking after disabled parents who lose £70 a week. Wait for the disabled ex-soldiers' complaints. All attention has been on cancer sufferers, but many with MS, motor neurone and other chronic disease lose employment and support allowance, too. Come 1 April, 280,000 disabled, both young and old, will have their benefits cut.

The government claims that these amendments cost £1.8m – which is less than honest, since it's spread over five years. DLA is cut by 20% – a random sum. That means no funds for transport or daycentre fees, just as council social services are withdrawn. This is only half the sum the DWP is pledged to cut from disability benefits. More is yet to come.

People on DLA being transferred to the lesser personal independence payment will have new tests, by their GP and by outside assessors, repeated regularly. That costs a staggering £675m to administer. Not surprisingly, Atos is backing off applying for this work after criticism of the company's work capability assessments, through which 38% of those appealed at a tribunal are overturned (and some die shortly after being declared fit). Citizens Advice wants Atos fined for every error.

Cameron is about to be reminded that most people are nicer than he thinks. Voters think two sensible things about benefits: citizens needing help should be well cared-for and the healthy should be deterred from malingering.

British Social Attitudes shows how the pendulum swings. In power, Labour increases benefits that have shrivelled miserably in Tory years, and most people approve – until gradually fearing their generosity may be exploited. When Tories are in power, people who voted to have scroungers scourged grow kinder when they see what cuts really do to the sick and unemployed.

Watch the public mood turn. These cuts are unlike anything ever seen before – and people didn't vote for savagery: they voted for a Cameron in sheep's clothing who promised general wellbeing, letting the sunshine in and care for disabled children.

Politicians promising perfect solutions to the benefits conundrum risk disappointing voters. William Beveridge never got it right, either. His clever idea was to make people feel everyone had contributed through national insurance, so the poor deserved support in times of need – good political cover for enormous redistribution. There was no NI fund and governments can cancel contributory "rights" at any time, as now with ESA. Hardly anyone knows which benefits are a right, earned through NI, and which are means tested, for need. Contributions created a notion of a moral bond, but it was always bogus. NI is just another tax, which governments dare not abolish to raise income-tax rates instead.

The eternal dilemma has been argued in the same terms since the Poor Law 1601: how does the good society care for everyone without creating "dependency" on that goodwill? Once, punitive workhouses were the answer, but they proved expensive as well as cruel, so it was cheaper to pay the poor in cash; anxiety about moral hazard was the price.

That's the choice, more or less generosity, stricter or gentler policing of people on the margins. Look at a post office queue on benefit day and see how many people inhabit a twilight zone of semi-disability, borderline odd, barely coping, and unlikely recruits by any employer. People's circumstances are complicated, and so the benefit system must be too, as it tries to grade levels of need as fairly as possible. Beware politicians promising simplification.

If not these cuts, then what, the government challenged the Lords rebels yesterday? The answer is to cut almost anything else before picking on the disabled. Bank bonus season is upon us – ripe pickings? This, too: before cutting 20% from the disabled, remember that a one-off windfall of 20% from the £4 trillion wealth of the top tenth could pay off the nation's debts at a stroke. But maybe we'd rather cut help for stroke patients.

Twitter: @pollytoynbee

• This article was amended 18 January 2012. The original stated that "Atos is backing off applying for this work after criticism of the company's work capability assessments, through which 40% are overturned on appeal". This has been corrected.