If this was meant to be the “Brexit election”, it has not lived up to its billing. The defining issue of our times – Britain’s departure from the EU and its consequences – was ignored in the early stages of the campaign and resurrected only after Theresa May’s poorly received manifesto unsettled her party. The real story about Brexit will begin to be told only after the election is over.

But this campaign has been illuminating all the same, revealing the motivations and priorities of our political parties and the scale of the change with which they must grapple. There is not much that unites the Prime Minister and the leader of the opposition but both would agree that the political and economic consensus of the past 25 years was rejected in last June’s referendum, which for many Britons was less about voting to leave the EU than sticking two fingers up to a remote and complacent political establishment.

Jeremy Corbyn is a determined and resilient campaigner and he has conducted a spirited campaign. The Labour manifesto has many popular (even populist) tax-and-spend policies: free tuition fees, nationalisation of the railways and the mail, more funding for schools and the NHS, more funding for the police, and so on. All of this would be paid for by higher taxes on those earning £80,000 a year or above and by raising corporation tax. He has defended his idealistic programme with passion and (mostly) good humour.

However, in bitter truth, and in spite of its uptick in the polls, Labour has fought a largely defensive campaign: seeking to bolster its core vote while failing to reach out to the constituencies the party once held and would need to win again if it had any chance of returning to power.

Mr Corbyn does not seem to understand why 1970s-style state socialism has been rejected throughout the world. He is uncomfortable with the institutions of the British state. He is instinctively against the Western powers and struggles to support the multilateral institutions (such as Nato) that created the postwar liberal world order and prevented another world war.

One of the enduring weaknesses of the liberal left is a sense of moral piety: we assume that our values are superior, that we care about the weak and the vulnerable more than the other side does. Indeed, many people on the left believe that the Conservatives are nefarious, which, in effect, condemns the millions who vote for them.

Another weakness of the left is an addiction to crisis. The Labour Party is dying, we are told. It has lost its sense of historic purpose. It will collapse if it loses on 8 June.

This is nonsense. This dismal general election campaign has shown that there is a deep desire for an alternative to the Conservatives in England and Wales and indeed to the Scottish National Party in Scotland. More than ever, Britain needs a purposeful and unified main opposition party, ­especially as it negotiates its fraught departure from the ­European Union.

A vibrant democracy depends on strong opposition, as we argued in our widely noticed issue of 31 March. Labour has always been a plural party, never in thrall to a particular section or faction. Labour is a patriotic internationalist party: the party of Nato and the United Nations. It was the postwar Clement Attlee government that commissioned Britain’s independent nuclear deterrent. Labour is the party of the great social reforms of the late 1960s – reforms that legislated against sexual and racial discrimination. Labour is the party of the National Health Service, the welfare state, the minimum wage, comprehensive education, the state academy schools programme and Sure Start.

Above all, Labour’s historic mission is to redress the power of capital and defend the labour interest for the common good. The mission of the Conservatives is to be the party of the moneyed interest and to defend the market state.

“A large constituency of working-class voters,” said the Harvard philosopher Michael Sandel in a New Statesman ­interview with Jason Cowley last June, “feel that not only has the economy left them behind, but so has the culture, that the sources of their dignity, the dignity of labour, have been eroded and mocked by developments with globalisation, the rise of finance, the attention that is lavished by parties across the political spectrum on economic and financial elites, the technocratic emphasis of the established political parties.”

The rise of Corbynism is a response to these feelings. But a leader who cannot command the support of his parliamentary party is no leader at all. This is not to dismiss those who voted for Mr Corbyn in two leadership contests: his candidacy in 2015 unlocked something long repressed on the left and his rivals for the leadership offered nothing but a barren, technocratic politics or a return to Blairite progressivism. Mr Corbyn promised to erase the shame of the Iraq War and liberate the party from the tortuous triangulations of Ed Miliband.

We have never supported Jeremy Corbyn’s leadership or Corbynism. But this is not a Conservative or a Liberal journal, nor has it ever been one, even though J M Keynes was our chairman in the 1930s. Today, our politics are liberal, sceptical and unpredictable, in keeping with our founding mission, which was to campaign for a more just society.

And yet, for all our criticism of Mr Corbyn, we do not want to live in a one-party state, in England or in Scotland. And we desperately want effective opposition, in England and Scotland. (Labour is in power in Wales.) We do not want a return to the politics of the 1930s, when Labour had been routed, and the country experienced hunger marches and the rise of fascism. And we want all traditions of the Labour Party to be represented in the leadership and shadow cabinet.

Britain does not have a presidential system. When we cast our vote at an election we do so for an individual candidate in a particular constituency. Many admirable Labour candidates are struggling to hold on to their seats – including, at random, Jon Cruddas in Dagenham, Holly Lynch in Halifax, Jess Phillips in Birmingham Yardley, Alison McGovern in Wirral South, Peter Kyle in Hove and Wes Streeting in Ilford North – because of the invidious position they have been put in by the party’s leadership. So when you cast your vote on 8 June remember that you are voting not for a party leader but for an individual candidate. That distinction matters.

A Tory landslide victory will only embolden the free-market fundamentalists, self-satisfied politicians such as Boris Johnson, and the virulent right-wing press. The best instincts of Theresa May and her estimable co-chief of staff Nick Timothy, who has thought deeply about the causes of inequality in Britain, are coming under attack from a coalition of libertarians and Cameroons. The Prime Minister’s panicked U-turns raise doubts about her ability to take on the Tory right, the press and the party base (as she will need to). A campaign designed to showcase her strength has ended up highlighting her flaws.

Mrs May and Mr Timothy are offering a critique of the failures of capitalism, but from the right. It is silly to describe them as “lefties”. But we should take them at their word when they say, as they do in the much-maligned Conservative manifesto: “We do not believe in untrammelled free markets. We reject the cult of selfish individualism. We abhor social division, injustice, unfairness and inequality.” Believe them: but then hold them to account. Challenge them to prove that their actions are equal to their words.

As for Labour, if Mr Corbyn loses on 8 June, he should resign. The party will need to learn lessons from politicians such as Sadiq Khan in London and Andy Burnham in Manchester, elected mayors who have shown how to win popular mandates. It will need to disavow faddism and create a social contract in which responsibilities are as important as rights and a job is not an end in itself, but a means to a life of dignity and fulfilment. Instead of seeking to govern a country that does not exist, Labour should renew itself by attempting to understand the British people, who they are and what they want. As George Orwell wrote of Britain in 1941, anticipating the transformations to come, “it will change out of all recognition and yet remain the same”.

It is this enduring sense of intergenerational and institutional wisdom, the desire to change yet also to conserve, that is the defining characteristic of the British nation. This is the invisible thread that binds the nation and creates the spirit of fellow feeling we have witnessed in Manchester in recent days. In upholding its best traditions, but finding new ways to serve the common good, Labour must change and yet remain the same. Only then will it win again.