A year ago I was still holding to two opinions about the medium-term future for the British Government and its Official Opposition. The first was that the great danger for the Conservative Party at the next general election was that it would win, but win very small. The second was that unless Gordon Brown had the nerve to go early to the country last year - when he would win - then any general election was most likely to be delayed until 2010.

After the week just past I see reason to revisit both judgments. Office could take the Tories by surprise much sooner than they think, catapulting them into power with a dangerously fat majority and a dangerously thin manifesto. Mr Brown's leadership could simply fall apart, and there is a fringe in the Conservative Party that might take this as some kind of a mandate for their own obsessions.

On Thursday in Cambridge an undergraduate from the Clare College Politics Society asked me when I thought the tipping point for Gordon Brown occurred. Had it been this week, with his 10p tax-band U-turn? Last October with the on-off general election? Or had it been some minor but ludicrous evidence of his disturbing disjunction from reality, like refusing to sign the European Treaty alongside his European counterparts?

I replied that tipping points in politics are usually retrospective constructions, but that my own guess was that Mr Brown's fate was sealed by last autumn, when the Prime Minister missed his chance for a personal mandate from the British people.

But I am losing confidence in what I had supposed was the corollary: that because there would never now occur a moment when Mr Brown could count on winning, he would simply keep going until his administration hit the constitutional buffers in 2010. I am no longer sure Mr Brown's Government will be capable of carrying on. Two more years of public and private disintegration may prove impossible to maintain.

John Major, at the lowest point of what was for him a very low time, was able to summon enough courage, and rely on enough respect from colleagues, to put his leadership of his parliamentary party to the test, and win. Mr Brown has already sunk below that. Were he tomorrow to precipitate a Labour leadership election it is likely one or more Cabinet colleagues of calibre would stand against him, and most uncertain (I would say doubtful) he would win. A leader who has never received, and now dare not ask for, his own party's or country's support, stands vulnerable to whatever gust of history may strike him.

Who can know from where that gust may come? None of Mr Brown's travails so far was widely predicted, and the next may well not be the local and London elections on May 1. The Conservative Party now has more to lose than gain from these. Winning will not be surprising news; and if Boris Johnson fails to win the London mayoralty, this could almost be painted as a great victory for Labour.

But those of us who maintain our long-held judgments about Mr Brown's utter incapacity for office need no more trouble ourselves with wondering what will humiliate him next than we need to study tide and weather charts to determine the moment when the incoming waves will breach the walls of a sandcastle. The castle is made of sand: it will yield. Gordon Brown is a vacuum: he will implode.

The implosion, however, will be ugly. Mr Brown is unlikely to go quietly. He may be mad but he's quite used to being mad, he's been mad for a long time, he doesn't see it, and on some ghastly level the prognosis is stable.

So can a leader with neither need nor desire for an election be dislodged? Yes. Parliamentary parties are composed of human beings, and human groups don't always act in their own collective self-interest, especially in times of panic when desperate individuals try to save themselves by breaking from the pack. The best prospects for the Parliamentary Labour Party, though dismal, are to hold its nerve, bite its tongue, cleave to the leader it has and try to keep him upright and on his feet. This should minimise the scale of defeat next time. The party's second-best prospects would lie in dispatching Mr Brown tomorrow, decisively and cleanly, and hoping to anoint a successor.

Labour's worst prospects lie in leaving Mr Brown in place but sniping at him and undermining him until he almost literally cannot stand up any more. It is this third prospect, the worst of all worlds, that looks to me likely. So are the Tories ready yet - and perhaps quite soon - to take over?

My answer is an emphatic no. There is still no real shape (though plenty of intricacy) to their plans. The manifesto picture resembles one of those ambitious jigsaw puzzles we received as children for Christmas and threw ourselves into with initial enthusiasm. By Boxing Day night what was visible was a large frame, floating within it several small islands where the assembly had gone swimmingly and a tiny patch of intense detail had emerged. Here was a flower; there a cloud; there a bunny rabbit; and somebody's entire hand and one left ear could be discerned in the corner, piece-perfect. But what was the whole picture of? What was the story?

Some of the best people on the Conservative front bench are responsible for these interesting patches of detail. But ask what the Tory party as a whole exists for, and from all those expert draughtsmen comes a collective clearing of throats followed by an apologetic murmuring about being modern, caring and compassionate. Meanwhile, the people volunteering memorable answers, tunes you can whistle, are the right-wing nutters, xenophobes, reactionaries and over-simplifiers: numerically a minority, but noisy and assured, with clear and simple songs to sing.

Their heyday was the hounding of John Major. They've had a thin time of it since, as poll after poll has suggested they were part of the Tories' problem. But they're still there, waiting and hoping.

Until recently they have been waiting for David Cameron to fail. But now comes a greater terror. Some of them are waiting for him to succeed. And it may be sooner, and success may be bigger, than the party has dared believe possible.

So before the Tory Right kid themselves that the coming collapse of a Labour Government has anything to do with them, and before so much as the hint of a suggestion arises that a big election victory for the Conservative Party would be intended by the electorate as a mandate for reactionary government, it is vital that Mr Cameron and his allies fill in the rest of that jigsaw with shape, colour and meaning: with a bold, sharp, popular story of their own choosing. There may be less time in which to do this than some of them think.

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