John Rentoul: No, a yes vote is not good for Brown. Yes, a no vote is good for Blair, too

The Prime Minister has slipped a disc. This is a rather unimportant event, although it may perform a public service by warning us that gyms are dangerous places to be avoided if at all possible. But there is no denying our fascination with intimations of the mortality of those set over us. We like to be reminded that our leaders are mere flesh and bone, and, in this case, gristle. It allows us to speculate about the weakness of the human body as a fulcrum for great turning points of history. My good comrade Mr Watkins hypothesises about the implications for Tony Blair's length of tenure in office on the opposite page.

Just as interesting though, is speculation not about Blair's health, but Gordon Brown's. He too, is only flesh, bone and gristle. What would happen if Brown fell under a bus? That is a question that shines a bright and unexpected light on the shifting substructure of British politics. We all know what would happen if Blair, in the quaint formulation of the Labour Party rule book, became "permanently unavailable" to serve as leader. Brown would become prime minister, probably without the trouble of a leadership election. There would be adjustments of tone and, at the edges of things, of policy, but political life as we know it would carry on.

If Brown disappeared, however, the political landscape would change dramatically. The question of the future direction of the Labour Party would be thrown open again and at least half of the members of the Cabinet would be canvassed as Blair's successor. Hypothetical history is a popular genre, from "What if the Germans had won the war?" to "What if Pontius Pilate had pardoned Jesus?" It is entertainment with a serious edge, because if it is done well it allows familiar events to be seen from a different angle. Hypothetical current affairs can be just as informative. The extent to which Brown already dominates politics can only really be seen clearly by imagining his absence. He underpins the stability and longevity of this government, because it is easy to imagine him winning back enough disillusioned Labour voters to carry the next election.

The trouble with such imagined possibilities is that they are either about things that didn't happen, or about things that are unlikely to happen in the foreseeable future. In a week's time, however, we face the perfect hypothetical: two starkly different futures, both more or less equally likely. Next Sunday the French people vote in a referendum on the European constitution and no one knows what the result will be. The implications of a Yes or a No will ripple out far beyond France, having a profound effect on British politics either way.

So much fun has this game become that British politicians have even suspended their "no hypothetical questions" rule. Tony Blair, Jack Straw and Douglas Alexander, the new Europe minister, have all been happy to answer hypothetical questions about what would happen if France votes No. But, because they don't normally answer this kind of question, they are not as disciplined as usual and have been giving slightly different answers. Instead of welcoming this new openness and willingness to engage in speculative futurology, the media have seized on the differences as evidence of splits. Oh, well.

These difficulties arise because there is no simple answer to the question. On the face of it, it would seem enough to say that, if the French vote No, there would be no point in having a referendum in Britain. The constitution must be ratified by all 25 member states, so if one fails to do so, the process ends. Blair was initially reluctant to say that, because it might sound as if that was what he wanted. (Especially when it is obvious that it is in fact what he privately wants.) But his reluctance also arose from the more elevated motive of knowing that Britain is likely to be asked to vote on something at some point. If the constitution falls, it is likely that a less ambitious document that makes some of the same changes to the EU structure will take its place in the next few years. Having conceded the principle of a referendum, Blair or his successor would have to hold one on any "constitution lite".

Of course, it would be politically convenient for Blair if the French did vote No. If it is difficult to persuade the French to vote for a document drawn up by Valery Giscard d'Estaing, it is much more difficult to persuade the British. Every time I look at the constitution I find new things in it to which to object. One of the victories for hard-headed British bargaining that I remember Jack Straw proclaiming was that the phrase "ever closer union" wasn't in it. I am not allergic to it, as anti-Europeans are. I know that the phrase was in the Treaty of Rome, and that we British have lived with it since joining 32 years ago. But Straw's claim was mere sophistry - worse, it was unnecessary sophistry that undermines the pro-European cause. The preamble to the constitution says that the peoples of Europe will be "united ever more closely". Then there are the symbols. "The anthem of the Union shall be based on the 'Ode to Joy' from the Ninth Symphony by Ludwig van Beethoven." (Perhaps they inscribed that in the constitution so that we didn't confuse it with the version by the Stranglers.)

It will be difficult to persuade the British electorate of the need to vote Yes when so many pro-Europeans support the constitution on a lukewarm "might-as-well" basis. But if it has to be done, Blair will try to do it. And he will try to do it the same way that he won the general election, by running a joint campaign with Brown. Again, imagine that the Chancellor is run over by a bus. The chances of winning a Yes vote are slim, but they are so much slimmer if Blair fights it on his own. A joint campaign worked in the election and it might work again. Hence the appointment of Alexander, the Chancellor's ally, as Europe minister.

Nor can Brown duck out of this campaign any more than he could out of the election. So he is not in the south of France, stuffing envelopes for the Yes campaign, hoping for Blair to crash to defeat in a British referendum next year, which is where some of his more naive supporters may want him to be. Brown has no interest in sharing responsibility for defeat, but he has a more specific reason for wanting Britain to vote Yes. Blair has to make it clear before a British referendum that he would stay as Prime Minister whatever the outcome - otherwise it would simply be a referendum on him. So Brown, like Blair, is hoping for a French No next weekend. The horrors of renegotiating the constitution - a negotiation that Brown might have to take over as prime minister - pale into insignificance against the horrors of fighting such a difficult referendum. Once again, Blair and Brown are, in what the constitution lays down as the motto of the European Union, "united in diversity".