Gordon Brown has always had something of a victim mentality. For years he was the plucky Labour underdog, fighting an all-powerful Conservative Party. Then, when Labour won power, he was the betrayed friend, denied the premiership by a double-crossing Tony Blair.

Even now, he (or his people) occasionally lapses into this way of thinking - he is the class warrior, oppressed by the wealthy Old Etonian David Cameron, or the brave captain, struggling to hold a course against global economic forces that are beyond his control - and now (how ironic this is) the long-suffering Prime Minister enduring a troublesome Chancellor. Some think it stems from a lengthy period pinned to a hospital bed and brooding after a rugby accident that left him with only one eye. The Big Clunking Fist is more aware of the punches raining down on him than of those he lands on others.

But as he enters the most difficult - and most crucial - month of his political career, Mr Brown can no longer play this card. He has the job he has always wanted, the keys to No 10, the run of Chequers and the prime ministerial bulletproof car. He is finally in charge and it is up to him to make his own luck. He has a relaunch, a reshuffle and some (conference speech) rhetoric through which to do so. There is no incompetent boss mucking things up for him, but also no convenient scapegoat to blame if it all goes wrong. The Macavity of politics has nowhere to hide. Already he is starting to look dangerously exposed.

Alistair Darling has provoked the wrath of Mr Brown with his suggestion that Britain could be facing the toughest economic times for 60 years. But it is the Prime Minister's proposals for dealing with the problems, rather than the Chancellor's analysis, that matter more to most voters.

The “economic recovery plan” of which Downing Street officials have made much will start today, with an announcement on housing. But privately, even some at No 10 say the plan is likely to be a damp squib because all the radical ideas have been filleted out. Instead of a stamp-duty holiday, there will be an extension of the shared-equity scheme for first-time buyers; rather than a winter fuel payment for all families there will be vouchers for poor households; in place of a windfall tax on the energy companies there will be an increase in the amount of carbon emissions allowances that groups are forced to buy. “The expectation management has been a disaster,” one Downing Street source admitted to me yesterday. “By talking up big plans, the spin-doctors just make the reality look small.”

Already No 10 is trying to pin the blame on the Treasury: “They have not been proactive and dynamic and creative in their thinking” one aide said. Certainly, Mr Darling disliked the whole idea of an economic recovery plan, believing that any measures should properly be included in next month's Pre-Budget Report. But there have been growing signs of wider tension between the Prime Minister and the Chancellor.

For some months Mr Darling has been giving a more gloomy assessment of the situation than Mr Brown - he refused a request from the Prime Minister to put a more positive gloss on the figures in his last Budget. He has also made clear that he should not be blamed for Mr Brown's decision to introduce a 10p rate of tax. Although the official line from both No 10 and the Treasury is that relations between Mr Brown and Mr Darling are good, privately insiders say they are dire. “It's desperate,” one Downing Street source says. “There has been a breakdown in the relationship between the Treasury and everyone else. I seriously doubt whether Alistair will be able to continue as Chancellor. What he did was absolutely appalling.”

A limited reshuffle - focused on merging the Scotland, Wales and Northern Ireland jobs into a single constitutional secretary role - had been pencilled into the No 10 diary for this week. Mr Brown is being urged to postpone it and carry out a wider reshuffle involving Mr Darling. It would be wrong of the Prime Minister to pin the blame for his and the country's difficulties on the Chancellor - and foolish for him to sack someone who knows where the political bodies are buried. But already Mr Brown has been forced to reassure David Miliband that his job is safe, after the Downing Street attack dogs went on the offensive against him. His political authority is starting to look dangerously weak.

Even in Downing Street, Mr Brown appears weak. When he first became leader, and was feeling confident, he declared his determination to widen his circle and recruit a new, and more modern, generation of advisers. Now he is retreating into his bunker again. The old Brownites are winning their vicious turf war with the new Brownites: Stephen Carter, the PR man brought in as the Prime Minister's director of strategy, is said to have been sidelined and Mr Brown has been talking to his old university friend Wilf Stevenson about becoming his strategy adviser.

Some at No 10 have even discussed the idea of giving Mr Carter a peerage and a ministerial job to get him “out of the way”, but, like sacking Mr Darling, this would be risky. The PR man was recently heard to say: “I know enough to be dangerous.” Friends say he has found his time in No 10 frustrating. “Stephen's disappointed in Gordon,” one told me. “Gordon brought him in to change things but he hasn't let him. He was taking focus group research to Gordon which pulled no punches and was demanding radical change to his style, tone and language but Gordon wouldn't listen. The guy is incapable of changing.”

Mr Brown is tempted to blame others for his predicament. But as a leader he is the master of his own fate. Or is he, as another chancellor said of a different prime minister, in office but not in power?

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