By rights, Labour should be on course for a landslide win on 8 June. The manifestos of the two main parties are now out, and Labour promises lots of things that people really like, while the Conservatives are offering bitter medicine that will especially hurt those who turn out in big numbers: the old. Surely voters will pick the party of sweet over the party of sour.

Labour's manifesto: analysis of the key points and pledges Read more

Just look at the two documents side by side. Labour pledges free school meals for all, free university tuition fees and free wifi on trains. (The word “free” appears 39 times in the manifesto.) There’ll be more police officers and more houses, as well as rail, water, mail and electricity out of the grasping hands of profiteers and owned by the public. Pensioners will keep their triple lock and their winter fuel allowance. Public sector workers will get a deserved pay rise, and there’ll be an end to the employment tribunal fees that too often render workers’ protections meaningless. Labour will spend £250bn on a national infrastructure that sorely needs it. Who could say no?

Especially when you look at the miserly Tory plan. Tuition fees will remain in place, as will the rapacious privatised utilities. The elderly will lose that automatic help with energy bills and see the security of their pension downgraded. What’s more, if the old need care, they’ll have to burn up the value of their home to pay the bills, right down to their last £100,000. Labour demands no such sacrifice, promising instead a generous extra £8bn for social care.

Given all that, surely it would take a special kind of national masochism for Britain to choose the Conservatives over Labour. Voters tell pollsters all the time that they want, say, to renationalise the railways or end zero-hours contracts: one party says it’ll give them what they want; the other refuses. How can there not be a resounding Labour victory?

And yet those same polls that show support for individual Labour policies also show an electorate readying to deliver precisely the opposite verdict at the ballot box. How do we explain a country that is poised to spurn Santa and his big bag of goodies and instead willingly embrace Scrooge?

May’s motive in channelling her inner Ebenezer is uncomplicated. She wants a mandate

Theresa May’s motive in channelling her inner Ebenezer is uncomplicated. She knows she’s going to win, so she wants a mandate – permission to make the moves she believes will be necessary over the next five or even 10 years in government. For that, she needs to shake off the commitments made by David Cameron in 2015 that were designed simply to win an election (and which he doubtless expected to bargain away in a coalition agreement). May’s manifesto is proof that you make very different promises if you know you’re going to have to keep them.

But why are the public not put off by her stinginess? The answer is rooted in Labour’s past – and has grave implications for its future.

The coming rejection of an apparently popular platform will be a brutal reminder of a central truth about politics, that what matters is not just the product you’re offering but your perceived ability to deliver it. It comes down to credibility. Voters don’t believe Labour has a hope of fulfilling its promises.

Play Video 4:46 Labour’s latest election broadcast produced by Ken Loach – video

This is not a new problem. Labour lost in 2015 for similar reasons. Though this is billed as a Brexit election, and though austerity and the deficit are now mentioned rarely, this election still stands in the shadow of the 2008-9 crash.

Now many of us may well insist, to our last breath, that the crash was made on Wall Street not Downing Street, and that if anything, Gordon Brown handled a global crisis with great skill. But, thanks in part to the relentless message discipline of Cameron and George Osborne, the settled view is that Labour messed up by spending too much. As Populus managing director and former Conservative strategist Rick Nye puts it: “The morality play of the noughties is that we borrowed more than we could afford, there was a reckoning and we don’t want to go back.” To repeat the old but effective Tory metaphor, Labour crashed the car and don’t deserve to be given back the keys.

You’ll note that I have not even mentioned the name of Jeremy Corbyn. That’s because this is a Labour problem, not just a Corbyn one. It would have bedevilled Yvette Cooper or Andy Burnham too. It predates even Brown, with roots in the 1970s, the IMF crisis and the “winter of discontent”. Unfair it might be – tirelessly pushed by a rightwing press, most certainly – but a near fixed point of British politics is the assumption that while Labour’s heart is in the right place, it cannot be trusted to run the economy.

Consider what it took for Labour to win in 1997. The Tories had to trash their own credibility by presiding over an economic crash, in the form of Black Wednesday; the economy then needed to become sufficiently buoyant that voters felt able to risk letting Labour have a go; and finally the party had to don Brown’s fiscal hair shirt, promising it had learned its lesson and would never again be profligate with taxpayers’ money.

Only then did the electorate allow Labour control of the Treasury. So the party always begins with a huge, historic credibility problem that it has to work triply hard to overcome. Corbyn didn’t create it – but there’s no hiding the fact that he, John McDonnell and Diane Abbott have made it much, much worse. The sheer scale of the largesse in the manifesto, promising item after item, has only fed the perception of fiscal incontinence.

General election 2017: Tories called 'utter hypocrites' for wanting winter fuel payment cuts for England but not Scotland – as it happened Read more

Abbott belongs in that list because of her car-crash LBC interview, in which she had no idea how much Labour’s planned 10,000 extra police would cost. Nye and others have seen the polling and focus groups which reveal that this encounter is the one event of this election campaign that’s truly cut through, reaching even those who barely follow politics. The hackneyed metaphor, designed to tap into pre-existing fears about Labour, became real. “Her sums literally did not add up,” he says.

It means that, come the next election and the one after that, the work of reputation reconstruction will be all the tougher. Future Labour shadow chancellors will have to be even more tightfisted than Brown was in the mid-90s, just to prove their worthiness for office. It’s not that any one item in the 2017 manifesto is unworthy. It’s just that, especially when taken together, they represent the kind of offer you can make only once you’ve earned the public’s trust.

It’s not that surprising, when you think about it. Consider the advice that a glass of red wine a day is good for you. If a doctor says it, you’ll accept it. But if the same advice comes from an alcoholic, you’ll hesitate. Labour aspires to be entrusted with the nation’s health and wealth. But the country first needs to be sure that its hands are not trembling.