Even as his troubles snowballed at the height of winter, the prime minister was on bullish form over dinner when he turned to his favourite subject, the economy. Northern Rock was a trial, he admitted, and the international outlook was indeed bleak but in no way was he downcast.

As chancellor, hadn’t he seen off the Asian crisis and the Russian debt disaster of 1998? His then Tory shadow, Francis Maude, had warned of a “downturn made in Downing Street” but it had turned out to be “a false dusk”. Under his stewardship again, the economy had rebounded after the bursting of the dotcom bubble of 2000 and the terror attacks of 9/11. True, there were dark clouds on the horizon for 2008 but sunshine would return. In a year’s time we’d all look back and put these difficulties into perspective.

In his first inaugural address Franklin Roosevelt famously stated that “the only thing to fear is fear itself”. We are in a downturn, not a great depression, so our politicians won’t wish to emulate FDR’s heroic hyperactivity; but the voters do need to see their fears addressed in concrete ways. That is a challenge as much for David Cameron as it is for Gordon Brown.

Six months after Brown’s prophecy to me, the voters are finding it hard to put their troubles in perspective. They stubbornly refuse to join a Monty Python chorus of “Always look on the bright side of life”. The prices of their bread, milk, eggs, butter and petrol have soared; tax rises have eaten into their salaries.

Brown and the chancellor, Alistair Darling, are nothing if not consistent. As the housing market was throttled by an absence of credit in March, a Treasury economic working paper boasted that the UK had “the most resilient” economy in the developed world. Last Thursday, Marks & Spencer announced that its sales had gone south. This is the sort of news even economically illiterate Everyman relates to. In response, Darling delivered a relatively upbeat assessment that interest rates and unemployment were far lower than in the downturn of the early 1990s.

Try as he might to portray our difficulties as made in oil-thirsty China, the prime minister is thought by voters, rightly or wrongly, to be accountable for the mess because of his decade-long tenure at the Treasury. They are not listening to excuses – in fact they aren’t listening to a word he says at all right now.

That is the problem with overhyping your success as the best economic results since “the Hanoverian succession”, and “an end to boom and bust”. Brown cannot escape his earlier boasts of monumental achievement as the monument crumbles.

Worse, the voters now look back on the period of stable growth he presided over at No 11 and put it down to smoke and mirrors. If the times were good for a decade that was because Margaret Thatcher had done the heavy lifting and later those clever Chinese churned out ever cheaper goods. The bust of the public finances was Brown’s doing alone.

Number 10 believes that if the prime minister can just get through another hellish by-election at the end of the month in Glasgow East, the voters will gradually come to see him as an authority figure once more. They will contrast Brown’s economic expertise with Cameron’s paucity of experience and see the error of their ways. Well, that’s the theory, folks.

Backbench Labour MPs who tore up the budget before the Crewe by-election and are seeking to tear it up again over fuel duty and car tax will not be so sanguine if the Scottish National party takes Labour’s 25th safest seat. Such a loss north of the border could be portrayed as heralding the break-up of the union and with it Labour’s ability to win another general election.

Number 11, stung by rumours of reshuffle – Darling is the only politician with ratings lower than Brown’s – rebuffs all attempts to set the chancellor against an interfering prime minister. With a lift of his quizzical eyebrows, Darling acknowledges that if his master wants to focus on the economy he is bound to feel a bit squeezed.

Privately he is scotching rumours he might be moved and has repainted the walls of his flat at No 11 to his taste to show he means to stay. But his team lacks bottom. Many senior civil servants have left the Treasury or moved with Brown to No 10. As for his ministerial lineup, Yvette Cooper is showing some fight as chief secretary, “Kitty in the City” Ussher has her admirers but is just back from maternity leave and Jane Kennedy is only about par.

Brown’s own long service at No 11 may be held against him in the polls but it stands him in good stead against rivals.

Sure, MPs doubt his leadership and communication skills – deputy leader Harriet Harman’s secret chats with small groups of backbenchers unearthed more discontent with the leader than the economy – but however bad things are none of his younger rivals has been tested in the Treasury’s fires.

In a downturn, David Miliband, James Purnell or even a cunning old fox like Jack Straw would find it hard to stand the heat. That is why the older Blairites, unreconciled to the PM despite talk of a rapprochement, can’t find a candidate to rally round.

The party that can draw on a first division of economic talent, is, surprisingly, the Liberal Democrats. Apart from the reassuring figure of Treasury spokesman Vince Cable, a former chief economist at Shell, other frontbench heavyweights – Chris Huhne, David Laws and Steve Webb – have a wealth of practical expertise in finance, economics, pensions and taxation law that the government and the official opposition struggle to match.

Cable has played the part of Old Testament prophet well. Long ago he thundered that easy credit, personal indebtedness and the housing bubble would lead to a crash. Nor is he short on answers. On Monday he will advance an imaginative proposal to increase a hundredfold a government scheme to encourage housing associations and local councils to buy up property which cannot be sold in the current housing crash. At one stroke the poor will be housed, building companies rescued and the banks will lose their worst collateral for debt. I do hope the Lib Dems won’t mind when yet another of their ideas is stolen.

For the Tories, the shadow chancellor George Osborne won his spurs by proposing the popular inheritance tax cuts that helped to scuttle Brown’s election plans last October. Brown and Darling were induced to copy him on both inheritance and a tax on nondomiciled residents.

Bad imitation really is the worst sort of flattery: the voters didn’t respect them for it, and the City looked at their version of nondom tax and capital gains revision and snorted “half-baked”. Indeed, Darling has earned himself the nickname of Mr Magpie for stealing Osborne’s policies, while the Financial Times has paid tribute to the shadow chancellor’s innovatory proposals for reform.

Lately Osborne has successfully created a narrative around the idea that Brown’s splurge on public spending has left no money to reflate the economy in hard times. His plain talk of “no rainy day money to mend the roof” is intellectually underpinned by the European Union and the OECD in their criticism of government extravagance. But as the voters become distressed by their empty wallets, they would like to hear from Cameron as well.

If anything, the Tories suffer from too stark a division of labour: Osborne leads on the economy and Cameron on social policy.

Now in a week when there were four more knife killings, I wouldn’t advise the Tory leader to give up on his theme of “the broken society” – he will visit Easter-house, one of the most deprived areas in the United Kingdom, this week (it’s in the Glasgow East seat) – but he needs to seal the deal on the economy.

“I want people to know that Cameron has a good first in PPE from Oxford, is comfortable with numbers, budget deficits and macro-economic policy,” says an ally. In that case, he should stop lending his name to stodgy articles and let us see the evidence . The prime minister jeers that Cameron was a youthful special adviser to Norman Lamont when the pound decoupled from the European monetary system. So what? At the same age young Gordon was setting out his plans to nationalise the commanding heights of Scottish industry.

Brown needs to find his footing and Cameron needs to find a voice on the economy. That’s the contest that will now largely decide the next election.

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