Today the ugly battle in Westminster resumes. All eyes are on Gordon Brown - including those of the Opposition. At Tory HQ they do not know how the enemy might move. But having planned so long for a Cameron v Brown election, they are now forced to think through the alternatives.

The toppling of Mr Brown is still not inevitable, though the net is tightening. The stench of fear is overpowering. Most Labour MPs assume that the general election will be a rout. It would be the ultimate irony if Mr Brown's ruthless assaults on his political enemies over the years now prove fatal to him by depriving him of enough big figures to serve in his Cabinet.

On Wednesday, the departure of Hazel Blears to spend more time on politics struck me as less spectacular than the sight of Shaun Woodward sitting next to Gordon Brown at Prime Minister's Questions. If Gordon Brown really did offer Mr Woodward the post of Home Secretary, as I am told, that is a measure of true desperation. It is understandable that the ex-Tory MP who once employed a butler has a schoolboy crush on the man who represents his last hope of preferment. It is incredible that the crush is mutual. But this desperate situation is not a completely unmixed blessing for the Tories.

One man everyone will be watching today is Alan Johnson, the likeable ex-postman whose union ties and radical stance on tuition fees have won him respect on Labour's Left and Right. He is the bookies' favourite to succeed Gordon Brown, a view fuelled by the widespread belief that Mr Johnson is the Labour leader the Conservatives dread most. But is he really? One senior Tory told me yesterday that “Getting Alan Johnson after Gordon Brown would be like getting Michael Howard after Iain Duncan Smith. He'd give a better performance, and for one day in the election campaign he might give us a fright, but he can't win.”

The general view of those around David Cameron is that although Mr Johnson speaks fluent human, unlike Gordon Brown, after a brief honeymoon he would be exposed fairly quickly as a lightweight. That Mr Johnson lost Labour's deputy leadership contest to Harriet Harman, they say, suggests that he lacks the killer instinct and will be outmanoeuvred either by another challenger or, at a general election, by David Cameron.

Alan Johnson could be Labour's John Major - the unifying moderate. He could also play the class card with enough subtlety to score against the Conservatives' privileged front bench. His accent and background alone would strike a contrast, with no need to resort to the kind of crude anti-toff rhetoric that backfired so spectacularly in the Crewe and Nantwich by-election.

But I detect no terror at Conservative HQ. One influential figure in the bunker, Nicholas Boles, wrote in The Spectator in 2006 that Mr Johnson was the man the Tories most feared. Today he admits that Mr Johnson is still probably Labour's best bet, partly because “he looks like he's in politics but not obsessed with politics”. But he thinks it is probably too late for Mr Johnson to overcome Labour's fundamental weaknesses: the party is much more deeply split than in 2006, and can't agree on where to go.

At the next election, the Conservatives intend to stick to their strategy of presenting their party as the one of change, as opposed to “more of the same”. The last thing they want is for Labour to change. Yet it is hard to see how a Johnson candidacy could achieve a convincing generational or ideological shift. He might bring David Blunkett back into the Cabinet, but that would hardly mark a radical break with the past. He would still need to keep Ed Balls and the Milibands onside. Another senior Tory I have spoken to sees James Purnell as a bigger threat than Alan Johnson, because he has the brains and radicalism perhaps to come up with something genuinely new. But other Tories think Mr Purnell lacks charisma.

Any Labour leadership challenge is bound to unsettle Tories who have planned for so long for the battle against Mr Brown. The polls have repeatedly given David Cameron's Tories a bigger lead over Labour when Gordon Brown is leader. Mr Cameron knows that Mr Brown will keep trying to portray the Conservatives as evil cutters of public services, and Labour as the party of investment, even when the public can see plainly that Mr Brown has remortgaged the bare cupboard.

Mr Cameron wins hands down over Mr Brown on eloquence and sheer humanity. There is no question that it would make life easier for the Conservatives if Mr Brown stayed where he is. But Tories are no longer spooked by the prospect of a Labour leadership election in the way they were over the contest-that-never- came in 2007.

David Cameron has rightly called for an immediate general election, which is what the public wants. But would some Tories privately prefer Gordon Brown to limp on into the spring?

The generation around David Cameron has been deeply scarred by the wilderness years that followed John Major's excruciating, long-drawn-out demise. Although the interminable death throes were probably good for the country, enabling Tony Blair to trounce John Major, they were disastrous for the Conservative Party, which was weakened by the infighting, brown envelopes and other sleaze. Tory MPs want a general election now, knowing that politics is unpredictable and that you must take your chance when the odds look good.

But not all are as gung-ho as you might expect. There is a still a humility about some of those who saw their side repeatedly wrongfooted by Tony Blair. Some would not mind having a bit longer to prepare. And there is the hope that the longer Mr Brown blunders on, the longer Labour will take to recover on the other side of an election defeat.

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