After failing to save the world (again) at the G20 finance ministers' meeting last weekend, with the universal rejection of his international transaction tax, Gordon Brown is now left with fewer than six months to save his skin. Parliament prorogues this week in preparation for the final truncated session before the sands of time force the Prime Minister to face the British people.

By my reckoning – assuming the general election is on 6 May 2010 – we are 177 days away from Mr Brown's eviction from Downing Street. That means 15 more question-time exchanges with David Cameron, one more Queen's Speech debate, a pre-Budget report, an actual Budget – and probably at least another 50 letters to be written by Mr Brown to the relatives of servicemen killed in Afghanistan.

Labour MPs facing defeat and MPs who are standing down either voluntarily – or involuntarily as a consequence of the expenses scandal – have no more than 70 parliamentary sitting days to enjoy the comfort of the green benches. They will complete six more expenses claims forms before their final pay-off cheque of up to £65,000 is banked. On the plus side, they need only contemplate the grim ritual of sending official Commons Christmas cards and House of Commons whisky to constituency worthies once more. This time next year a few lucky ones will be swathed in ermine but the rest will be selling one of their two homes – hoping that Sir Ian Kennedy will allow them to keep the capital gain.

In the meantime the Prime Minister will be trying to "Rebuild Britain's Future", the theme given in June to the draft legislative programme. Any element of surprise has been taken away from next week's State Opening ceremony. We already know the speech will focus on creating training, skills and apprenticeship opportunities for young unemployed people, although this was undermined by weekend reports of a secret cut of £350m to this budget.

But the rest of the pre-announced list of bills, including an Energy Bill, the Digital Economy Bill and yet another Policing, Crime and Private Security Bill, hardly seem likely to make any electoral impact on voters this side of the election. And as for the Constitutional Renewal Bill, I suspect that the House of Lords will see to it that much of this is sunk without trace by polling day.

Mr Brown's hopes no longer lie in legislation and will be determined by economic indicators now largely outside his and his Chancellor's control. We know that VAT will increase back up to 17.5 per cent and that unemployment will continue to rise. The possibility of a technical return to economic growth might lift the gloom momentarily but, with no room for manoeuvre, the opportunity for a vote-winning Budget giveaway will be denied.

Of course, it is entirely possible that some completely unanticipated event might provide an opportunity for Mr Brown to play to some of his strengths, although given his record so far this really would be clutching at straws.

I've often wondered what the effect of a huge unexpected national or international crisis, occurring in the middle of an election campaign, would have had on Margaret Thatcher, John Major or Tony Blair. Luckily, all our recent elections have occurred without unknown events crowding out the contours of the hustings. Certainly Thatcher and Blair would have excelled in such situations, reinforcing their poll leads already established at the start of their campaigns. If some awful terrorist event had erupted during the 1992 election, Major would probably have gained in prestige, following the Gulf War a year earlier. Equally, I suspect, however, that some untoward national event during the 1997 campaign would not have saved him, regardless of how well he might have handled the situation. The US experience in the banking crisis, during last year's presidential election, solidified Obama's poll ratings to an even greater extent. The die, once cast, merely reinforces and rarely changes voters' perceptions.

With what cards can Mr Brown now shape the political weather? Only surprise. He could relinquish the reins in his party's interest at the turn of the year. But this would only bring a marginal benefit, even if an iron discipline on an agreed cabinet successor ensured a transition to a new leader within days of his resignation. Suppose Alan Johnson or whoever were to emerge, complete with white smoke, Pope-like, in the early days of January, along with the immediate announcement of an election date. The novelty of a changed political landscape might prevent a Labour meltdown but would still be unlikely to prevent the inevitable Tory victory.

Only the timing of the election remains Mr Brown's wild card. He could follow the example of John Major by calling an election the day after an early Budget. If he wanted a short campaign, by avoiding Easter, 25 March remains an outside bet. But, frankly, Mr Cameron would still hold all the trumps.