In August 1992, I found myself playing football on the lawn of Stansgate, Tony Benn’s home in Essex, with Jeremy Corbyn and his son. I had interned for Benn and, along with other graduates of his basement office, was helping him move his archives. Corbyn had driven up for the day from north London with his young family. As we kicked the ball and talked politics, it never occurred to me that it was Corbyn, not Benn, who would come to represent the highest peak of the left’s advance within Labour by becoming party leader. (Neither did it occur to me that, one day, I would be the director of his leadership campaign.)

Corbyn’s capacity for exceeding expectations should now be the least surprising thing about him. Yet new shocks to the system keep coming. Since the election, his approval ratings have soared as Theresa May’s have plummeted.

It’s necessary to understand what has happened. History is a combination of objective reality and politicians’ choices. Corbyn’s leadership is an answer to questions that have been posed since at least the start of this century. Over that period, Labour has shed support from both middle- and lower-income voters to smaller parties, eradicating the certainty of the two-party system. Between 1997 – when we won 13.5 million votes – and 2010, we lost 4.9 million votes. The decline was marginally reversed in 2015, when our vote rose by 0.7 million. At the same time, the party has undergone a slow ideological evolution, rarely fully and openly expressed, concerning whether a social-democratic platform could win.

The achievements of Tony Blair’s government are unarguable. When, for example, a left-wing shadow education secretary, Angela Rayner, talks movingly about the impact of Sure Start on her life as a young single mum, she speaks for millions. Yet the case against Blairism’s limitations is well rehearsed: its reforms did not challenge the fundamental structure of the economy after neoliberalism. First with Gordon Brown and then more clearly with Ed Miliband, the opportunity for a revival of social democracy was offered. Notably in Miliband’s proposal for an energy price freeze, we saw a pointer of ways to think differently about the economy. A corrective to the Blair period was under way within Labour, but it fell short of creating a popular movement and lacked the clarity needed to spark one.

After our 2015 election defeat, party members were not prepared to return to the old ways. It’s easy in the current climate to forget that uncomfortable feeling many Labour Party members experienced whenever a Labour minister was on the Today programme: wondering what you would have to endure with clenched teeth before you heard something you supported. On Iraq, civil liberties, trade unions, tuition fees, public services and even privatisation, the left in British society had become alienated.

As the 2015 Labour leadership campaign began, it appeared that there would be a return to that period. From rejecting trade union funding to a tough turn on welfare, old and unpopular lines were being revived. If a politician taking that approach had won the leadership, the party would have haemorrhaged even more support, but Corbyn’s campaign, which swiftly gathered momentum, offered a way out of the cul-de-sac.

The decision of Labour members and supporters in 2015 to opt for change in the form of Corbyn has been vindicated. Britain’s economic recovery since 2008 has been slower than that experienced after the 1929 crash. Even many who believed that there was little alternative to austerity following the financial crisis want something more to hope for. Voters’ increasing concern about the state of the NHS is just one expression of this. In such circumstances, there is little room for right-wing social-democratic solutions, let alone pure liberal ones. Our times demand more fundamental answers.

The election result in June – when Labour won 3.5 million more votes than it did two years earlier and secured 40 per cent of all ballots cast – has made a left-wing, social-democratic government a genuine possibility. The New Statesman’s Stephen Bush has pointed out that Labour needs a swing of just 3.5 per cent to win a majority of one; a swing of 1.4 per cent from Tory to Labour would put Labour at the head of a minority government. Though Labour’s advance was not only the result of the support of the young, the “youthquake” is real and represents a great opportunity if the party can hold on to these voters. Significantly, Labour was far ahead among the young working class.

Outside Scotland, a largely two-party system has asserted itself and the Conservatives are on the ropes. Labour should step up its opposition to the Tories and to a “hard” Brexit, as a way of protecting living standards and making them central to the national debate. If Corbyn can sustain the support he garnered in the election and win those who didn’t vote for him, he could exercise hegemony at all levels of the labour movement. A crucial factor in achieving this will be the building of a modern left, with the encouragement of an intellectual infrastructure around what has happened. However much of a ferment has taken place, a great deal of the labour movement’s debate is quite tactical. Labour needs to inject more heft into the culture of left-wing political thought. The election manifesto should be used as a launch pad for a wider discussion about the story we are telling about Britain and Labour’s contribution to it.

Read more: John Harris on how Labour must offer hope for the future

The chance to lead with real consent brings with it a responsibility for all parts of the party to do so in a culture of open-mindedness and mutual respect. There are many voices out there – from the trade unions and the voluntary sector to the arts and academia – who want to feel part of a modern left, and all must be engaged.

Labour’s performance in the general election seals the deal after at least 20 years of discussion. The question now is not whether but how to put democratic socialist ideas into practice.

Simon Fletcher was the director of Jeremy Corbyn’s 2015 leadership campaign and his chief of staff