What has happened to the Labour Party? In 2010, the likes of David Miliband and Jack Straw lent their nominations to the token left-wing candidate, in the interests of widening the debate. Diane Abbott duly stood up to the plate, and was duly eliminated when the first round of votes were counted. Yet when centrist Labour MPs attempted to repeat the trick in 2015, they unwittingly unleashed Corbynmania on the world.

Many have taken this as a sign that the Labour Party has lurched to the left, carried away by an influx of naïve teenagers, entryist Trots, and mischievous right-wingers hell-bent on rendering Her Majesty’s Opposition unelectable. But there is a far simpler explanation. In 2010, as in 2015, three prominent New Labourites stood at one end of the spectrum, with a token left-winger at the other. The major difference is that, this time round, there is no longer anyone standing between these polar extremes of Labour politics. The difference is that there is no Ed Miliband.

In their rush to distance themselves from their previous leader’s legacy, the Labour leadership hopefuls left a huge expanse of ideological space unoccupied. The mainstream candidates retreated to the parliamentary party’s New Labour comfort zone, leaving no-one to offer a middle path between the old left and the not particularly new right. And that is why many Labour Party members now find themselves forced to give Corbyn’s candidacy serious consideration.

Yes, the Corbynmaniacs are extremely loud and extremely visible, particularly if you venture below the line on the Guardian website. But at constituency party meetings up and down the country, there are many who are simply Corbyn-curious: people who are deeply undecided, because they are not well-represented by any of the candidates on the ballot paper. These are the people who voted for Ed Miliband, and who were drawn to the party that he created.

It is high-time that the mainstream candidates reappraised the legacy of their last leader. Miliband gained the Labour Party a hearing among many who had been turned off by the New Labour years: by the war on Iraq, the financial crisis, and the widening gap between the richest and the rest. He acknowledged that mistakes were made. He stood up to the energy companies’ cartel, calling into question pricing practices that ripped off millions. He forged a new consensus on press regulation. His plan to transform productivity among the lowest skilled by increasing the minimum wage is being put into practice by none other than George Osborne. Ed prevented British forces from rushing into an ill-considered intervention in Syria. And he accomplished all this while sat on the Opposition benches.

We know now that these achievements were not enough to win him the election. And on their own they would not be enough to get his successor into Downing Street either. But they offer the promise of a more inspiring political vision than a watered-down version of the Conservative’s election-winning manifesto, or the virtuous impotence of perpetual opposition. The real question is: what needs to be added to these sound foundations, in order to create an election-winning proposition with broad-based appeal?

There were undeniably gaps in Miliband’s manifesto. He never seemed able to address the concerns of business owners, large or small. He had little to say about private sector growth; he did not seem to have a clear sense what Britain’s place in the global economy should be, let alone how we should get there. Banning zero-hour contracts without showing where better jobs were going to come from sounded more like a threat than a promise to people at the bottom end of the labour market. When all was said and done, many voters simply did not see how the Labour Party was going to deliver the sustainable prosperity necessary to drive up living standards and fund public services.

The real tragedy, however, is that providing such an account would have been completely compatible with the rest of Miliband’s platform. It would have cost him nothing to acknowledge that the cost of living crisis was not solely a problem for the state to solve, but was something that properly functioning markets could also ameliorate. At the same time as oligopolistic energy companies were pushing up prices, the novel business models of Aldi and Lidl were sending shockwaves through the supermarket sector, saving hard-pressed families hundreds of pounds. It was this supermarket price war, coupled with Saudi Arabia’s decision to flood the market with cheap oil, that helped to give Osborne’s long-term economic plan the illusion of success just as the election was looming into view. Had Miliband championed private sector competition as a possible solution to the cost of living crisis, he would have shown that state intervention was a last resort, and demonstrated that the energy price freeze was not the thin end of a red wedge.

Similarly, there were gestures in the direction of a sound plan for private sector growth: mutterings about repositioning Britain for a global race to the top, as opposed to the slash and burn approach to public investment taken by the Tories. At times, Miliband seemed to be combining Blair’s emphasis on education, education, education with a strategic, light-touch industrial policy designed to build upon Britain’s strengths in sectors such as high-tech manufacturing, pharmaceuticals and renewable energy. This could have been a radical vision that united business leaders and trade unionists alike. But all too often, these ideas were lost in the din of divisive rhetoric, drowned in a misguided attempt to show people whose side the Labour Party was on.

There is untapped potential in this centre-left terrain, which – judging by both this leadership election and the last – is home to the majority of Labour Party members and supporters. And, so far as the current contest is concerned, the ideological space once occupied by Ed Miliband is still there for the taking. Jeremy Corbyn shows no desire to claim it, determined instead to push further left than Miliband would have thought electable or desirable. Liz Kendall has disavowed it entirely. Andy Burnham has staked out several left-of-centre policies, but Labour supporters who remember him running as the Blairite candidate in 2010 struggle to see him as a credible heir to Ed in 2015. Perhaps unfairly, his campaign smacks of opportunism rather than conviction.

That leaves Yvette Cooper, the least ideologically defined of all the candidates. As such, she still has a chance to appeal to the Party’s centre-left majority. Her passionate defence of the achievements of the last Labour government would not be incompatible with a clear-sighted recognition of its flaws, were she so inclined. Obviously, if this is not what Cooper believes, then there would be no point in her faking it. But unless someone reaches out to the Corbyn-curious, there is a risk that Labour’s summer fling with the unassuming left-winger will blossom into something far more serious.