What a cornucopia of delights is here. The leaked Labour manifesto is a treasure trove of things that should be done, undoing those things that should never have been done and promising much that could make this country infinitely better for almost everyone.

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No, this is not a repeat of the “longest suicide note in history” 1983 manifesto. There is no reprise here of the killer pledges that caused the party to split back then – pulling out of Europe, calling for unilateral nuclear disarmament, protectionist exchange and import controls, or nationalising pharmaceutical, building materials and many more industries. Reluctantly, no doubt, here is a pledge to keep Trident and spend Nato’s required 2% on defence, an essential backstop against those who regard Labour as perennially weak on national security.

Celebrate all that is urgently needed – repairing the NHS, social care and school funding. Restoring working rights to the bogus self-employed, and those on zero hours, and bringing back collective bargaining, without which pay has fallen sharply since the 1980s as a share of GDP, while profits and top pay take a soaring share. Bringing back mail and rail to public ownership, as contracts expire, not full-scale energy nationalisation, but one state energy company in every region to make the comparison. Investing in a million homes. Borrowing £250bn over 10 years for urgently needed infrastructure is good capital investment we can well afford. Free tuition fees, educational maintenance allowances for poor families, free school meals – all these should please.

If the country would welcome and consent to all of this, it would undoubtedly be a better place to live and bring up children, and probably more prosperous too. How welcome to see Labour reject a “no deal” Brexit, and insist on staying in the single market and the customs union. If the country would agree to restoring the cruellest benefit cuts and to an open, “fair” migration system, we would be a nicer, kinder, more optimistic nation than we currently seem to be. But we are not that place, not right now in Brexit Britain.

Politics is the art of the possible. It really is an art, one of the most skilled, if undervalued and unjustly despised. Politics is the art of persuasion, step by careful step bringing enough people with you. Item by item, almost all of this manifesto could be popular – but that’s not enough without the solid groundwork laid. How Labour rebuilds economic trust and cajoles the country into a less fractious, individualistic, often xenophobic, state of mind will be its long future task.

A leader’s first job is to counter their party’s negatives from the start. Theresa May, bidding for leadership, pretended not to be from a nasty party for the privileged few, but working for the “just about managing” whom she nonetheless skewered in her first budget. Labour always has to work a hundred times harder to counter its perceived negatives against the blast of an overwhelmingly Tory press. To counter the untruth that it “maxed out its credit card” last time in power, Labour needs to prove absolutely iron-fisted fiscal probity. A manifesto that is a banquet of glorious spending will be a very hard sell. Voters may like the look of much of the feast, but they suspect the food may be a mirage and they fear the bill. However, it may also encourage them to complain at May’s pitifully empty pauper’s table.

Hold your breath and wait for the blowback from the Tories and their bully press. Headlines in the Mail and Telegraph already blare that it will “take us back to the 70s”. There are few figures in the leaked manifesto, but though Labour claims everything is costed, recent embarrassing ignorance on numbers by Corbyn’s top people raises the worry that their accounting may not survive ruthless broadcast interviews or the neutral scrutiny of the IFS. Let’s hope each item is water-tight, and all can be paid for only by corporations and the top 5%, with a risky pledge of no tax rises for anyone else.

Here’s the real peril: if these costings fall apart, then the Tories can claim it proves all these policies are unaffordable. Already the rightwing drumbeat pretends that falling funding proves the NHS is an “unsustainable black hole.”

Some pledges look expensive. If, as Aneurin Bevan said, “the language of priorities is the religion of socialism”, then gifting large sums to students from wealthy families, free school meals to those who can well afford them, or triple-locking pensions to the rich retired may add to a sense of extravagance. In the current climate, where pollsters find voters (wrongly) resigned to austerity as a necessity, all this will require a well-trusted, inclusive, great persuader – but Jeremy Corbyn is not that leader. Even popular policies risk failing in his hands for lack of credibility. To paraphrase St Paul, though he speak with the tongues of men and of angels, but has not credibility, he is become as sounding brass, or a tinkling cymbal.

Unreasonable perhaps, but suitability as prime minister overrides everything else. Listen on doorsteps or to what people say to Labour leafleters on the street and the view of Corbyn is fixed. Pollsters find it repeated in every focus group: “IRA … Trident sub with no nukes … no national anthem … no shooting armed terrorists … Marxist … knows more about Havana than Hastings … a protester not a leader.” There was no chance he could wipe out a lifetime of backbench rebellion to become the healer of a divided nation.

Liking elements of his manifesto won’t reinvent Corbyn: that’s why most MPs knew he couldn’t become a winner and overcome his old negatives. The die was cast long ago. As Ben Page of Ipsos Mori has always said, very few people change their minds during an election campaign. His language is not reassuring to many: he threatens “a reckoning” against the greedy, allowing the Mail to charge him with class war.

But as Corbyn is Labour’s leader there’s no point now in producing some mean little manifesto with minuscule timid pledges. It’s quite right to go large and please Labour people with a dream of what might be: this is no manifesto for Marxist revolution, whatever the Tories will claim. And never mind who leaked it – it has probably done Labour a favour in extra publicity. This manifesto rightly challenges the monstrous cuts that are shredding the entire public realm and it will force May to respond. She will win, but as she dodges everything and everyone, she looks less strong and stable by the day, and here’s a reminder of all that she avoids.

The long-term danger is that good policies in this manifesto will wrongly go down in history as “rejected” by voters – when all they will have rejected was Corbyn.

• This article was amended on 25 February 2019. Labour’s 1983 manifesto did not call for pulling out of Nato as an earlier version said. Rather it called for the abandonment of nuclear weapons.