Two years ago, in the thick of Jeremy Corbyn’s first Labour leadership race, I wrote that he “has stormed through the crash barriers of contemporary politics as if they weren’t there” and warned the Tories not to be smug about facing him across the despatch box.

Always trust your instincts. In the months that followed, I became convinced that the new Labour leader was, in practice, turning his party into a political club, rather than a prospective government, and courting electoral oblivion. I should have heeded my initial impression.

Were a second general election held on Thursday – the first anniversary of Theresa May’s appointment as prime minister – Corbyn’s chances would be excellent. At the Durham Miners’ Gala on Saturday, he offered to “help these Tories out of their nightmare” and invited the PM to go to the country again, as soon as possible.

He is right to pursue the strategy of the sleepless campaigner, harrying the Conservatives in marginal seats such as Chingford and Woodford Green, where Iain Duncan Smith’s majority has been reduced to 2,500. In politics, momentum is hard-won and easily lost. Precisely because the political class is exhausted by almost four years of referendums and elections, the potential dividends of stamina in the next few months are unusually high.

May attempts to reassert grip over Tory party amid talk of challenge Read more

I still think that May should have pre-announced her resignation on 9 June, and that her £1bn deal with the Democratic Unionists is nonsense on very wobbly stilts. But the Conservatives have made a clear collective decision to play it long, on the assumption that Corbynmania will fade, that the Tory message will be broadcast more effectively, and that the PM’s incumbency will be reinforced by geopolitical sandbags such as President Trump’s promise on Saturday to deliver a post-Brexit US-UK trade deal “very, very quickly”.

That said, there is an alternative Tory interpretation of “playing it long” that ascribes a, shall we say, lesser role to May. Andrew Mitchell, who is close to David Davis, is reported to have described her at a private gathering as “dead in the water”. This is not an uncommon sentiment, though it leads different mutineers to different conclusions about when May should go and who should replace her. What unites them is a determination to postpone the next election for as long as possible.

In response, Corbyn needs first and foremost to be realistic about the likely form, content and brutality of the next campaign. Expect May’s new communications director, Robbie Gibb, to bring grip and imagination to the task. Money is pouring into Conservative campaign headquarters, new electoral themes are being framed and tested, and previously muzzled ministers are being actively encouraged by No 10 to tear into Labour. There is nothing so ruthless as a cornered Tory.

Corbyn’s continuing campaign needs to be no less ferociously focused. As a fiscal conservative, I have argued and continue to argue in favour of deficit reduction. But there is no doubt that in the word “austerity” he has found a shorthand for all the government’s failures – real and perceived. It now connotes much more than an economic strategy, evoking, as the phrase “winter of discontent” did for so many years, a much broader sense of unease. It’s society, stupid.

With this theme as his principal propulsion, Corbyn now has to ensure that every fibre of Labour’s being is strained towards looking like a government-in-waiting. Arguments about the deselection of MPs, and the extent to which the party should be a broad church, are a serious distraction from that central ambition. It was a mistake not to invite Chuka Umunna, Yvette Cooper and other senior non-Corbynite figures into the shadow cabinet. Such a gesture would have bolstered his position, a sign that he was strong enough to be magnanimous. To win, Labour needs the Conservatives to be seen as the party of splits, in contrast to its own unity of purpose.

The party needs 64 more seats to secure a Commons majority. Winning 30 more than Ed Miliband in 2015 was no small achievement, a triumph over pollsters and pundits alike. But political history shows how hard it is to cross the final bridge that separates opposition from government.

Consider, for example, how far Labour had to travel between 1992 and 1997. David Cameron was Conservative leader for 10 years before he won his own party majority. There is no such thing as “one more heave”. If Labour is to form the next government, it must now chase remorselessly voters over the age of 55, non-graduates, and those who do not yet trust the party to implement Brexit.

Above all, Corbyn has to turn his movement’s face unequivocally to the future. Labour has always looked best when it is promising with plausible urgency to remodel a broken system that the Tories no longer understand. There must not be a whiff of nostalgia or restorationism about the party’s project.

This is primarily a question of mindset rather than ideology. Every single thing that every single Labour figure says between now and the next election has to be rooted in the here and now and in the coming challenges of the 21st century: automation, housing, social care, new forms of inequality, modern pluralism. The lovestruck public will stop chanting Corbyn’s name soon enough and ask if he is ready for a serious commitment.

The left’s weakness has been its belief that there is an inexorable direction to history, that triumph is preordained

All of which means that the party’s conference in Brighton in September must be a rigorous campaign launch rather than a carnival of celebration. The left’s great weakness has been its belief that there is an inexorable direction to history, and that its triumph is preordained. If the last election has a single lesson it is that politics has rarely been so volatile or prophecy so powerless. Corbyn is tempting fate by declaring, as he did last week, that there is “no question at all” Labour would win another election held “straight away”.

If the party prevails, it will be because its leader and his followers have learned that victory is not the child of historic forces but of human agency, incremental advance and the capacity to see past doctrine to the needs of the hour. The extent to which they grasp this distinction will do much to decide whether Corbyn is the next prime minister – or just the chanted name of a single summer.