It's not very tasteful, but I didn't start it. Derek Simpson, the co-curmudgeon of the super-union Unite, started it. In his interview in last weekend's Independent on Sunday, he talked of what would happen "if Gordon fell under a bus".
It is not the best analogy. Not only is Simpson joint general secretary of the bus drivers' union, as Unite incorporates the old Transport and General Workers' Union, but the likelihood of a prime minister being run over by any form of public transport must be low. Still, we get his drift.
Simpson went on to think aloud about Ed Miliband: "I think he's quite good; if I had to pick a leader for the future." Which was quite wrong, of course, as well as being simultaneously unhelpful to the Energy and Climate Change Secretary, to his brother, the Foreign Secretary, and to Ed Balls, the Schools Secretary. Ed Miliband is a potential candidate – if he can push aside his elder brother – after Labour loses the election; Simpson was getting ahead of himself, as well as exposing his lack of confidence in Gordon Brown.
The fact is that if Gordon did fall under a bus right now, Alan Johnson would take over. There is no need to go into the details – I've done that enough already: Labour would probably still lose the general election, but not as badly.
That is not my theme this week, however. As we enter the conference season, Simpson has performed an accidental service to the cause of political understanding. The "under a bus" test may not be tasteful, but it is a way of illuminating the underlying structures of politics. Let us apply it to each of the parties in turn. Labour emerges surprisingly well for a party supposedly exhausted by 12 years in office. And the Liberal Democrats, whose conference opened yesterday, also pass the test. If Nick Clegg were run over by a yellow and blue Bournemouth bus, Vince Cable, the most trusted man in British politics, could take over without missing a dance-step. The party even has a second back-up, in the form of Chris Huhne, the home affairs spokesman who ran Clegg close for the leadership two years ago.
It is when we apply the test to the Conservative Party that it reveals something quite different. It is no secret, of course, that the Tory recovery in the past three years depended heavily on the personality of David Cameron. But the "under the bus" test shows just how much of a one-person party the Tories have become. Who would take over? Would it be his "very able deputy", as Cameron described William Hague a few days ago? An interesting phrase, because Hague is not formally Cameron's deputy. Cameron could give him the title of deputy leader of the party, but he chooses not to, fobbing Hague off by calling him "my deputy in all but name". Hague did not have a happy time as leader, up against Tony Blair when Blair could do no wrong (not that he did any wrong later, but you know what I mean). He may think that his time had finally come, but his party would be bound to wonder what the voters would make of him. He just didn't cut it last time, and it would feel like a great step backwards.
But who else is there? Not George Osborne, the shadow Chancellor. He is clever, certainly, but has not yet acquired that magic stuff that comes across well on television, the substance described as personality, charisma or gravitas. It was a tiny incident last week, but as he moved a box away from behind a podium at which he was about to speak he called it "the Sarkozy box", which was hardly a diplomatic way to refer to the leader of a friendly country with whom he may have to deal next year.
David Davis, whom Cameron beat in the run-off among party members four years ago, has shown himself to be of unreliable judgement, resigning to stand against himself in a by-election on an issue that few remember. Or there is Liam Fox, who narrowly missed the run-off, possibly because Cameron made a cynical pitch for his Eurosceptic followers. Fox has hardly been prominent as defence spokesman over the past four years; and nothing has diluted his reputation as an astringent rightwinger.
Kenneth Clarke, although he will be 70 next year, could even be back in the mix, especially if the Irish shut down the European issue in their referendum next week.
Beyond that, the ranks thin out quickly. Michael Gove, the education spokesman, once borrowed Boris Johnson's line, saying that he had more chance of being reincarnated as an olive than of leading the Tory party. As for Johnson, he would have to get back into the House of Commons.
This little survey reveals how vulnerable the Conservatives are. Of course, the present Government consisted for a long time of the personality cult of Tony Blair. It was his personality that sustained New Labour in office, as was proved by what happened after he stepped down.
If that is being overdependent on the personality of the leader, give us some of it, the Tories might say. But there are dangers for them that Labour did not face. One lies in the lack of preparation. It is often assumed Blair came to power as an innocent, spending time learning on the job before working out – too late – what he wanted and how to get it. This is a caricature of a government that delivered devolution, a Northern Ireland settlement, a minimum wage, primary schools reform – all of which Labour had worked on before the election.
Another lies in the scale of the challenge facing any government that is elected next year. New Labour came to power in a golden economic climate. If Cameron becomes prime minister, the situation could not be more different. When such difficult decisions have to be made – with not a single large cut in public spending identified and no tax rises whatsoever – is it enough for a new government to rely so heavily on the winning personality of its leader?
John Rentoul's blog is at independent.co.uk/jrentoul