Will a Miliband bid bring Johnson in?

The man himself continues to deny it, but speculation about a David Miliband challenge to Gordon Brown for the Labour leadership continues unabated. Political betting guru Mike Smithson has today become the latest pundit to predict a Miliband candidacy, following last weekend's Sunday Telegraph tale that John Reid would be giving the Environment Secretary his backing.

But here's a question no-one seems to have asked as yet: what impact will a Miliband challenge have on other wannabe leaders who have thus far ruled themselves out of challenging Brown - ostensibly on the basis that he is the best candidate, but secretly because they don't think they can beat him?

Look at it this way. So long as Brown remains the only serious candidate, and overwhelmingly the most likely winner, there really is no great incentive for someone like Alan Johnson or Hilary Benn to challenge him. Far better to settle for the deputy leadership and (hopefully) a big job in the Brown Government.

But the moment that situation changes, and Brown faces a serious challenge which could theoretically result in him being defeated, then by my reckoning, all bets are off, and all earlier denials of interest so much hot air.

Such a scenario would present a particularly acute dilemma for the fifty-somethings Johnson, Benn and Peter Hain were the 40-year-old Miliband to be that challenger. The current consensus is that if Miliband does stand, he will at the very least establish himself as the heir-apparent, and could even win.

But that, of course, is the last thing Alan Johnson wants. He doesn't want the Labour leadership to "skip a generation" - at least not just yet. He wants to be deputy so that he can slip effortlessly into Gordon's shoes if the next election goes belly-up. The same may apply, to a slightly lesser extent, to Benn and Hain.

Hence my hunch is that if Miliband does stand against Gordon - and I'm still by no means convinced he will - he won't be the only one.

The "ultras" - Reid, Charles Clarke, even Blair himself - may all line up behind him, but he won't get a clear run. And at 40, with other, vastly more experienced people for the Labour Party to choose from, why on earth should he?

* Historical footnote. Similar calculations about whether a challenge to an established frontrunner could create a domino effect causing others to throw their hats into the ring also operated last time round, in the 1994 leadership contest.

One of the principal though lesser-known reasons Brown didn't stand on that occasion was that had he done so, it would have brought his old rival Robin Cook into the race.

With the support of the left and the likely second preference votes of Margaret Beckett and John Prescott, Cook would in all likelihood have come second, ahead of Brown, establishing himself as the de facto No 2 in the Labour pecking order.

People who knew Brown and Cook of old in their Edinburgh days have told me this was something Brown would have wanted even less than to see Blair leading the party.