So approaches the one day of the year when even the most mottle-faced Blairite can dredge up some sympathy for Jeremy Corbyn: budget day. Of all the jobs dumped on a leader of Her Majesty’s opposition, responding to the budget ranks among the hardest.

First you must squirm against the benches as the chancellor rapidly unloads a barrage of forecasts and targets on something that isn’t even your specialism – then you have to get up and argue forensically that they are merely the spume of a fantasist, while your plans are gloriously sound.

The good news for Corbyn tomorrow is that the first half of his job is quite simple: it has never been easier to criticise George Osborne. The bad news for the Labour leader is that he doesn’t yet have a lot to say on how his party would do better. And that will turn out to be a much more serious deficit.

Let’s start with the case against Osborne – which can be made almost entirely by quoting the chancellor back to himself. Consider this: at his spending review at the end of last November, Osborne bragged about the UK’s “strong economy”. Just six weeks later, as we stumbled into January, he was warning of a “cocktail of threats” and “the beginning of the decline” for the UK. It will be the same tomorrow, with Osborne using this gloom to justify making a further £4bn of cuts.

We were fine, runs this argument, but now things are getting a bit choppy so it’s time to shorten the sail. And it is bunk. Cast your mind back to last November. The financial pages were already full of market turmoil. The slump in steel prices had just tipped the SSI plant in Redcar into liquidation, threatening thousands of jobs, and more closures were being pencilled in.

Days before Osborne stood up to boast about his robust recovery, his former cabinet colleague Vince Cable had been on the front of the Independent warning of “severe economic storms” – which, given that he’d spotted the last crash, was a warning worth heeding. Meanwhile, Adair Turner, former boss of the City watchdog, had publicly described the UK as “dangerously unbalanced”.

In other words, your average London cabbie could have told you something bad was brewing. Yet granted £27bn extra by the Office for Budget Responsibility in its less than hallowed forecasts, the “iron chancellor” didn’t bank the cash and thus allow himself more leeway to meet his savings targets – but spent as much as he could.

And since Osborne is Osborne, and has never clapped eyes on a pudding that he didn’t think could benefit from an extra egg or two, he did so while bragging about his amazing recovery. Tomorrow marks the point at which he has to take back both the boasts and the cash.

It’s worth spelling all this out because it takes you to the heart of what’s gone wrong with Osborne in office. It’s not that he’s some crazed austerity-fetishist; it’s that he’s a complete chancer. He might bang on about his long-term economic plan, but he’s a short-term tactician. In one budget, he plans to shrink the state to its smallest since Orwell trekked to Wigan Pier; when that causes an outcry, he first attacks the BBC then switches tack at the very next budgetary event; when economists describe the new forecasts as a “roller-coaster”, the spending plans are changed again.

This is Osborne all over: he swears blind to be sticking to the course, even as he pulls off U-turn after U-turn. In opposition, he matches Labour’s spending plans – then claims they were rubbish. As shadow chancellor, he savages quantitative easing – then once in No 11, presides over a £375bn QE programme. He wants Britain to save more – except when he needs a house-price boom to jump-start a stalling economy. He promises a “march of the makers” until that fails, and then vows to lay on a northern powerhouse instead.

Even then, as his former Treasury colleague David Laws revealed this weekend, the chancellor couldn’t care less which cities actually form this powerhouse. After being lobbied by Nick Clegg to include Sheffield, the then-deputy prime minister came out of the meeting chuckling: “George is hilarious. He immediately suggested including Sheffield and just dropping Leeds.” Sheffield, Leeds: who gives a flat cap about the difference?

Corbyn is probably too gentlemanly to bring all that up tomorrow – but he should repeat the analysis of government infrastructure that he voiced this weekend. That shows, of all the spending on infrastructure on which work is actually under way, almost half – 48.6% – is going to London. The north-east is getting less than a penny – 0.8% – of every pound spent.

Some things Osborne does care about. Such as cutting tax on corporations, which has been one of the consistent themes of his time in Downing Street. You can bet that he is very serious about the pledges to raise the tax personal allowance and the threshold for higher-rate taxpayers.

If brought in before the next election, these two policies will cost £8bn. That is £8bn a year that the chancellor will be spending mainly to help the better-off. Meanwhile, he is set this week to make £1.2bn of welfare savings from people so disabled they need help to use the toilet.

If Corbyn wants targets on Wednesday, he has plenty to choose from. But the time is approaching when he’ll be rightly expected to say a lot more about how this Labour leadership would do things better.

If he wants an anti-austerity politics, a policy such as borrowing to invest is a modest step in the right direction. But the challenge facing Corbyn and shadow chancellor John McDonnell, or any future Labour leader, is far bigger: not just hashing out the old statist-Keynesianism of more spending on a high-tech future but talking concretely about what they would do right now about an economy that has cut out whole swaths of the country and its people – from Sunderland to Croydon.

This would be hard for any Labour leader – Ed Miliband abandoned the task pretty early on – and it is harder still for a man so comprehensively briefed-against and undermined as this one. But this time next year, pointing out Osborne’s gaffes and cruelties won’t be enough. Voters will expect some kind of an alternative. And if the world economy continues to slump, Corbyn will need to have one drawn up.