One-nation Conservatism, whatever it means, does not describe Boris Johnson’s performance at prime minister’s questions yesterday. He was all Bullingdon yahoo, full of triumphalist thuggery, spouting outrageous untruths with a bravado cheered to the roof by his own side. A broken Labour party could only look on in silence as its defeated leader soldiered on, his valid points vanishing into the bluster and braggadocio.

Take just one falsehood in Johnson’s stream of unconsciousness: when Jeremy Corbyn mentioned poverty, the prime minister said it had “diminished by 400,000”. The Resolution Foundation’s senior economic analyst, Adam Corlett, tells me he “struggled to find anything to back that up”. Instead, the figures show that according to one measure – the number of households having less than 60% of the contemporary median income – child poverty is up by 500,000.

Johnson didn’t understand Corbyn’s point on the effect of universal credit (UC) on low-paid workers: any Greggs worker earning just over £12,500 a year would see almost all of their £300 bonus taken away from them owing to a steep taper. Imagine imposing that 75% tax rate on high earners.

Corbyn ploughed on: what of the two-child benefit limit, one of the gravest poverty-creating policies? Yada yada yada, went Johnson: “I refer the right honourable gentleman to the answer that the British people gave to him four weeks ago.” Pre-election, he was more cautious. I hear reports from Labour MPs that questions raised with Tories have been greeted with a sotto voce, “80, 80, 80”, trumpeting their majority. Get stuffed, you lost, end of.

Is this a fading of poverty as a political issue? Corbyn’s stump speeches rattled on about rough sleepers, poor children, inequality, food banks, social injustice: the fear is that his abject failure has harmed the potency of those very issues. The frightening question is whether most voters don’t much care about everything that makes Labour party members of every faction keep on keeping on. Championing the underdog is in Labour’s DNA. The party itself is mostly middle class – even if they deny it – and inspired to do good for others, not themselves. That is a very different proposition from Labour’s foundation as a party of a mass working class rising up to fight for its own rights.

The change came gradually: the party of Harold Wilson was divided on the question of the poor, as powerful unions mostly represented well-paid workers, and were often reluctant to champion the genuinely poor and low paid. It remains the case that most unions representing better paid workers spend little time on the hard grind of recruiting those in greatest need of collective action – fast-food workers, care workers or van drivers. Meanwhile, latest figures show that some 84% of people work in the private sector, these days barely knowing what a union is.

“The many not the few” resonated fine with Labour supporters. But in Corbyn’s formulation it sounded like an old fashioned notion of the masses against a small cadre of bosses. The shape of society has changed: it’s more of a diamond shape – the majority (voters) in a fat middling with the poor a (non-voting) minority at the bottom, and a top soaring away from the rest. This election posed the question: how much does that middle care about the poor? Johnson seems confident that they don’t care very much at all.

Although many young working families in the middle of that diamond depend on universal credit top-ups, suspicion of idle “welfare” claimants vies with empathy for the needy. Voters in four elections have allowed the plight of the poor to worsen. Visit any food bank, stand in line at any Citizens Advice centre and you hear heartbreaking stories of cruel treatment from a wrecked welfare state.

This is one of Labour’s many great dilemmas. Tony Blair and Gordon Brown channelled hefty sums into tax credits, lifting a million pensioners and a million children out of poverty, improving their services and creating Sure Start. But they did it without talking much about it. They talked tough on benefit cheats to certify the valid claims of the rest, which may have confirmed the erroneous idea of widespread fraud. Even in their prime, they barely dared use their power to persuade voters of the value of social security.

The government is confident that not enough voters care, so their budget is unlikely to tackle poverty and repair the welfare state. But over the next few years, they may overreach. When austerity was generally accepted as a necessity, general belt-tightening meant extra-tightening for the poorest. Now Johnson has declared austerity to be over, pretending to splash the cash, will his government find it harder to justify a rising tide of poor children – now nine in every class of 30?

The yobbishness of his slapdash behaviour in the Commons yesterday was miserably depressing – but it may yield a seedling of hope that he will not be as adept at speaking for the nation as he thinks he is now, in this brief halo of victory.