Leonard Cohen's music is notoriously depressive but the singer does not consider himself a pessimist. “I think of a pessimist as someone who is waiting for it to rain,” he once said, “and I feel soaked to the skin.”

Alistair Darling - who last week attended a Leonard Cohen concert to cheer himself up - must feel the same. The heavens have opened since he became Chancellor. Rising oil prices, a falling housing market and the credit crunch have created the perfect economic storm. Mr Darling is a natural Eeyore - he prefers the shade to the sun - but there is no point in asking him whether the glass is half-full or half- empty because its contents have been emptied over his head.

At the Treasury these days it never rains but it pours. Yesterday it was reported that the Chancellor had dumped the tax on foreign profits, a “clarification” that follows U-turns on capital gains tax, the 10p rate and fuel duty. Last week, Mr Darling had to confirm that he was reviewing the fiscal framework on which Labour has, for more than a decade, based its reputation for economic competence. Perhaps the Chancellor found himself humming the words to Cohen's song One of Us Cannot Be Wrong: “The duty of lovers is to tarnish the golden rule.”

I wonder if that is how Mr Brown sees it. He was, by all accounts, less than pleased when he saw reports that the Treasury was considering dumping his Two Commandments, the golden rule and the sustainable investment rule - he preferred to move the goalposts so that the figures would fit.

The relationship between the Chancellor and the Prime Minister is one of the most intriguing in politics. The received wisdom is that Alistair is Gordon's puppet, sitting obediently on his master's knee mouthing the words that Mr Brown speaks.

The reality is more complicated. Mr Darling is more impressive than he sometimes seems. He has a calm, understated confidence, combined with a dry sense of humour. And he is increasingly willing to stand up to the Prime Minister. Cabinet ministers have noticed a shift in the balance of power in recent months. “Alistair is gaining confidence,” one says, “and that's a good thing; there should be tension between No 10 and No 11.”

There are no plates crashing down Downing Street, as there were during the TB-GB days. The Darlings still babysit on occasion for the Browns. But the Chancellor is starting subtly to differentiate himself from the Prime Minister. Last week he admitted to Alice Thomson and me that he had had rows with Mr Brown - “you'll have to wait for the memoirs to find out what they were about”, he said, an only thinly veiled threat. “Alistair's no puppet,” says a friend. “He's not going to allow himself to be the scapegoat for things that are beyond his control.”

There have, it is said, been tensions over fuel tax, alcohol duty and the Pre-Budget report, which Mr Darling was forced to rush out to prepare for an election that never took place. The Chancellor has let it be known that it was he who first spotted that disaster was looming over the 10p rate of income tax. One of the first things that he did on arriving at the Treasury was to work out how many losers there would be from the change. But for months, the Prime Minister refused to admit that there was a problem. By the time he did, it was too late - the Labour revolt had taken hold and the Government looked weak when it finally did act.

More recently, there have been differences between the two men over how to deal with the economic downturn. The Prime Minister has for some time been more reluctant than the Chancellor to admit how badly Britain may be hit by the credit crunch. I am told that before the last Budget Mr Brown tried to persuade Mr Darling to give a less gloomy assessment. The Chancellor now says that the slump has got worse and could last for years. Mr Brown remains more optimistic. He would never admit - as Mr Darling did last week - that the voters feel that they have been squeezed enough on taxation.

Although both men are Scottish, their backgrounds are different: Mr Brown, the son of the manse, was Labour from birth; Mr Darling, a public-school-educated lawyer, had a great uncle who was a Conservative MP. He is less tribal about his politics. At the Treasury, Mr Brown operated through his special advisers and a few trusted officials; Mr Darling is rebuilding the Civil Service machine. The Chancellor was shocked to discover recently that there are only three people left in the Treasury who have any experience of dealing with a recession. He is trying to recruit more.

There are those in No10 who are urging Mr Brown to sack Mr Darling. About three weeks ago, some of the Prime Minister's advisers were actively discussing appointing Alan Milburn as Chancellor to give an electric shock to the political and economic system.

The names of Jack Straw and Ed Balls have also been put into the frame. The argument is that when Mr Brown relaunches himself in September with a “new economic plan” he will need a new chancellor to help him. In fact the Prime Minister should strengthen Mr Darling's position and leave him to make his own decisions. The Government's reputation for economic competence has been undermined partly by the perception that the Treasury is being run from both No10 and No11.

Mr Brown would be foolish to remove his Chancellor - by doing so he would remove one of the few lightning conductors he has and leave himself exposed on the economy. He would also create a dangerous enemy. From Geoffrey Howe to Nigel Lawson, Peter Thorneycroft - and, indeed, Gordon Brown - angry chancellors, or former chancellors, are the most dangerous enemies that a prime minister can have. It is not only Leonard Cohen on Mr Darling's iPod - his teenage children have also introduced him to the American rock band the Killers.

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