Free stuff for absolutely everyone. What could possibly go wrong with that?

There is a spectre haunting this election campaign – and it is the last election campaign.

The Tories are desperate not to have a rerun of their 2017 experience when they started with a whopping poll lead only for everything to fall apart as the campaign progressed towards decision day. By contrast, the Labour leadership would be very pleased if this election were to imitate the last one, in which the party and its leader started out with absolutely awful ratings, but ended up performing sufficiently well to deprive the Tories of a majority and give renewed life to the Corbyn project.

This is the context in which to view Labour’s promise to provide the fastest full-fibre broadband to everyone. And for free. Gratis. Sign up and one of our engineers will be calling on you to install Corbyn Infinity Broadband some time before 2030. Terms and conditions apply. Offer expires on 12 December.

After a lacklustre and gaffe-strewn start for both of the bigger parties, this is Labour trying to revitalise its campaign with bold, look-at-me, vote-for-us, someone-else-will-pay pledges, a trick that worked for them in 2017. Labour’s strategists are entirely explicit in saying that they hope this “consumer offer to voters” will help diminish the poll deficit with the Tories.

To the credit of the party’s propaganda machine, the broadband pledge was landed effectively in the media and dominated the news cycle in a way no other announcement by any of the parties has done during this campaign. It will have pleased, not bothered, Labour’s leadership that there are furious arguments about its costs, feasibility and effect. The telecoms boss who attacked it as “broadband communism” was helping them out. If he’d had more wit and a more nuanced understanding of leftwing ideologies, he would have called it “broadband bolivarianism”. Boris Johnson also did them a favour when he called it “a crackpot scheme”. Better that everyone is talking about a Labour policy pledge that is going to be popular with at least some voters than that everyone is tugging at the loose threads of Labour’s contorted and muddled positions on Brexit, another referendum on Scottish independence or immigration, to name three of the topics in which the party had got into a hot mess in the preceding 48 hours.

That may help explain the rather curious timing of the announcement of Corbyn Infinity Broadband. You might have expected a pledge this splashy to be saved for manifesto launch week rather than sprung upon an unprepared world late on a Thursday night. This commitment was never previously agreed at a Labour conference and John McDonnell, the shadow chancellor, had previously said that British Telecom was “not on my list” of nationalisations. This strengthens my hunch that the policy was rushed out to distract from Labour’s difficulties on other fronts and as a diversion from its internal rows about the contents of its manifesto. Though many doubt that this broadband pledge will ever be fulfilled even if Labour wins this election, it is an excellent illustration of both the strengths and weaknesses of the Corbyn programme. On the positive side, the party can field some reasonable arguments in favour of the policy. It can be persuasively contended that the internet is not a discretionary consumer service, but a utility essential to the lives of everyone who has not chosen the life of the digital hermit. There is also a good case to be made that the private sector has done a poor and slow job of modernising Britain’s network and the state needs to step in to ensure that there is universal provision. Mr Corbyn, a man who is not always at his most accomplished when explaining policy, made those arguments well when he unveiled the pledge.

Early polling suggests that it has gone down positively with many voters. Because it involves another nationalisation, the creation of an additional state entity and new, if vaguely defined, taxes on tech giants, it will be called left wing. But it is not actually redistributionist. The greatest beneficiaries of free broadband would most likely be the more affluent households, which would get for nothing what they are already capable of paying for. So file this one in the same folder where we keep the pledge to scrap all student tuition fees: the folder marked Labour gifts to the middle classes.

Related: Andrew Sparrow's election briefing: Labour's broadband pledge dominates

It differs from Labour’s other proposed nationalisations in key respects. When a Labour government takes over the railways, the energy utilities, the water companies and the Royal Mail, all of which they propose to do, they have not said everyone will then get free train travel, free water and sewerage and free post, gas and electricity. These organisations will move into state hands, but you will still have to buy a ticket or a stamp or pay a bill.

With the broadband pledge, Labour says it would “literally eliminate bills for millions of people across the UK”. This is obviously designed to be the most voter-attractive element of the package that Labour costs at £20bn and industry experts say will come in at more like £100bn. The flaw in this pledge as a campaign device is that it will feed into the already widely held belief that Labour’s prospectus is less a carefully costed plan for Britain and more an ever-lengthening wishlist with little plausible indication of how it is all going to be paid for. No Tory has been as mocking of free broadband as Chris Leslie, the former Labour MP and shadow chancellor and once an aide to Gordon Brown, who joked: “Why not throw in free Sky TV? Free iPhones? Netflix and Xboxes all round?”

Labour’s election offer contains a bundle of free stuff, including more free childcare and free prescriptions for all. It will likely be tempted to add to them when the party next needs a diversionary headline. Very often, these promises poll well when tested individually. Why then, when the party appears to have lots of policies that many find appealing, is Labour trailing in the headline polls?

Probably because it is an old and usually reliable law of election campaigns that it is not enough to have policies that are popular. Commitments also have to be credible. You may like me more if I promise everyone in Britain a month’s free holiday on my super-yacht, but you will trust me less when you realise that I don’t have a super-yacht.

Labour has three critical problems with its many spending promises. First, a lot of voters tend to be instinctively suspicious that a party promising the moon will end up costing them the earth. Fairly or not, that suspicion tends to be higher when the party is of the left.

That handicap is more severe if the public has pre-existing low levels of trust in the party’s messengers. The shadow cabinet as a collective do not radiate reassurance that they would manage the public’s money as carefully as if they had earned it themselves. John McDonnell is the best of them when he does his impression of an avuncular bank manager. Despite the shadow chancellor’s efforts to project himself as a safe pair of hands, Labour is badly behind the Tories when voters are asked to rate the parties on “economic competence” and this despite the fact that the official statisticians have just confirmed that last year’s growth was the feeblest in a decade. It doesn’t help when the shadow chancellor falls out publicly with the shadow health secretary about how his proposed four-day working week would affect the operation of the National Health Service. If the party’s leading figures can’t agree how their policies would work in practice, and voters aren’t confident that the economy and their livelihoods are going to be safe in Labour’s hands, they aren’t likely to believe that the party’s promises can be delivered.

This is compounded by the sheer scale of Labour’s spending promises. It is true that all the parties are splashing around the cash at this election in the apparent belief that a promise a day keeps the voters at bay. I’m minded to agree with those who argue that this is most likely to deepen public cynicism. Labour is by far and away the biggest of the big spenders. The Tories shouldn’t bother concocting phoney estimates of how much a Corbyn-McDonnell government would add to state expenditure and borrowing because I think the country has already twigged that it would be a very big number.

That is the spectre from the last election that haunts this Labour effort. In case anyone needs reminding, the party lost in 2017. Eye-catching spending pledges and promises of loads of free stuff can grab headlines and energise the campaign and then not secure power. And without that vital commodity you never have the means to give away broadband or fulfil any of your promises. Not a single one.

• Andrew Rawnsley is Chief Political Commentator of the Observer