David Cameron received an unexpectedly easy ride in the House of Commons yesterday. If Jeremy Corbyn can’t even remember to quote the Tory former Work and Pensions Secretary’s devastating criticism of the Government’s welfare policy, you wonder if he is ever going to make a competent leader.

Of course it isn’t easy. But Labour really can't allow these opportunities to pass. In a halting and breathless response Mr Corbyn said a “huge hole had opened” in the Budget – but the gap was clearly on the front bench of the Labour Party. This was worse than Neil Kinnock's failure to capitalise on the Westland crisis after the resignation of Michael Heseltine in 1986.

Mr Corbyn allowed the Prime Minister to gloss over the damage. Lacking any nearby hoodies or huskies to hug, Mr Cameron announced the scrapping of the tampon tax instead. Then, citing measures like the National Living Wage, he congratulated his Government for being “a modern, compassionate, one-nation Conservative government”

Unfortunately, the cabinet minister with responsibility for compassion had already destroyed this claim. In his resignation letter, the politician in charge of welfare reform, Iain Duncan Smith, said that he simply couldn’t tolerate the unfairness of the benefit changes – not just in last week’s budget but in general.

He said Tories were targeting the most vulnerable in society “because they don't vote” yet were effectively handing out welfare to the middle classes and wealthy pensioners who do. We are not “all in this together”.

Now, a lot of people have pointed out that this was pretty rich coming from the minister who had presided over the so-called bedroom tax. Many believe Mr Duncan Smith was motivated less by compassion and more by a determination to shaft his political enemy, the Chancellor George Osborne.

But I don't believe it was just about personal ambition. Like a lot of old-style, patrician Tories with an armed services background, Mr Duncan Smith genuinely believes that it is possible to reconcile a market economy with social justice.

This strand of thinking is deeply ingrained in the Tory party and goes back all the way to Benjamin Disraeli and one-nation Conservatism.

The problem for the Conservatives in the 19th century was that the Reform Acts were giving the lower orders the right to vote. The party therefore had to appeal beyond its traditional constituency of landowners, clergy and merchants.

Disraeli revived the feudal concept of “noblesse oblige”, to argue that the wealthy had a paternalist obligation to look after the poor, just as the great estates had provided homes and (sometimes) schooling for the peasantry.

One-nation Conservatism allowed the Conservative Party to become the most successful election winning machine in Britain throughout the 20th century. It saw off both the old Liberal Party, which was much more ideologically capitalist, and Labour, which represented the organised industrial working class.

It did so, not because it simply duped the workers as Marxists believed, but because the Conservatives presented a coherent moral vision of an organic community in which the nation was united in a common purpose.

It may have been naive of working class voters to buy into this vision but they did. It confirms the enduring power of morality in politics. People very rarely vote simply with their class interests; otherwise capitalism would have been overthrown in the 1930s.

But nor is Britain an essentially Thatcherite political culture. That greed-is-good, no-such-thing-as-society mantra was imported from America, which dances to the beat of a very different drum in general elections. Mrs Thatcher's neoliberalism nearly destroyed the Conservatives in the 1990s.

This is why Iain Duncan Smith is important, and why the Tories are in real trouble over unfair welfare. It doesn't matter that Labour is led by a bearded old lefty with a charisma bypass. The voters know what Labour stands for even if they don't really believe it is competent at running the economy. Many would still back Mr Corbyn rather than live with a government that is – as Mr Duncan Smith put it – only interested in serving the better off.

The so-called “moderates” in the Labour Party, who want Mr Corbyn out, were growing worried about the prospect as they saw Labour rivalling the Tories in some recent opinion polls. But when you have a political advantage you have to press it home in the high court of public opinion: the House of Commons. Jeremy Corbyn let Mr Cameron off the hook yesterday and that won't be forgotten.