Labour lost in May because the party believed 2010 was its nadir. It seemed unthinkable that millions of people who had voted for Gordon Brown, in the midst of economic crisis, would abandon the party five years later. The Scottish polls seemed unreal and the UK polls disguised the truth. But in the event, while Labour won over a great many disgruntled former Lib Dem voters, it also lost millions of its own 2010 supporters, not just to the SNP, but to UKIP, the Greens and the Conservatives.

Above all, Labour lost its grey vote, with Ipsos MORI reporting that support for the party fell among the over-65s from 31 per cent to 23 per cent. These voters, former Brown and Blair supporters, sniffed the air and concluded that a vote for Labour was too great a risk. If Labour had secured their confidence and trust, it would be in government.

Now, the party has a mountain to climb. A recent Fabian report estimated that Labour needs to gain at least 106 seats in 2020 to win a majority, reaching deep into suburban and market town England. In this new context it would be an electoral disaster to veer wildly to the left, especially after a Labour defeat caused by lack of credibility and fear of the unknown. Indeed, the electoral facts against a big shift leftwards are now far stronger than before, just as many party members are flirting with the idea for the first time.

In 2015 the goal of uniting left-leaning voters offered a plausible path to victory, albeit one that failed. At the time the Fabian Society’s research endorsed Ed Miliband’s electoral strategy as well as his analysis of the nation’s social and economic challenges. But the political geography has now changed and Labour will only secure a majority by persuading people who voted Conservative this year to return to Labour in 2020.

A shift to the left would certainly doom Labour to defeat in the English and Welsh marginals, where there are far too few left-leaning Lib Dem and Green voters to make a difference. And there is no particular evidence that tacking leftwards would help Labour reconnect with the party’s traditional voters who have deserted it for UKIP and the SNP.

But nor can the party simply shuffle to the right, because it still needs to convince liberal urban voters, the lost working classes and the Scots. The party’s opponents will seek to divide this broad constellation of voters on lines of culture, values, and identity. Labour’s task is to unite them – and it must do so by showing it understands Britain’s future and can combine fairness and hope with competence and security.

This is the challenge for Labour’s new ‘big tent’ politics. The party must reach out in all directions, by sounding relevant, speaking clearly and with conviction, and looking like a credible government in waiting. Rather than a shift to the left or the right, the answer is a break from the reactive, managerial, muffled style of politics which Labour has been unable to shed since losing power.

The first step is to avoid despair. Today Cameron and Osborne are at their peak, but five years is a long time and political pendulums swing. The Conservatives will make mistakes, or be pulled from the centre-ground by their own extremists. Recession could return, and if it does, the left must be ready to pin its origins on our home-grown economic vulnerabilities.

So Labour must be prepared, as a competent, professional opposition to pounce, when ‘events’ shift the political weather. That will mean taking tough decisions in the short term, because the new leader must show that the party has listened and changed. Labour must be true to itself, but its aim must be to earn a hearing from pensioners and private sector workers, not its own activists.

The party must not refight the battle it has just lost, nor turn back to the nostalgia of 1945, 83 or 97. It will win again when it can show it has a story of the future; that it is the party of the 2020s. That means combining a deep understanding of the trends that will shape our lives – technology, inequality, ageing, climate, housing, tensions between global and local, the changing nature of government – with a hopeful account of Britain’s next chapter.

In this, Labour must not offer a story of risk and rupture. Instead it must prove that, in the uncertain world of the 2020s, only the left offers a credible version of stability and security. At the next election, people – of every age – must say, the real risk lies in not voting Labour. After Ed Miliband’s failure to land this message, a shift to his left will doom the party to even greater defeat.

A version of this article appears in the summer edition of Fabian Review.