Hartlepool will decide Blair's future

Yesterday Brown again proved why he is the natural successor to Blair. But this is not a new revelation. Everyone knows, and most Labour people still accept, that Brown is the leader in waiting. However, everyone also knows that Brown is extremely cautious - with the striking exception of last year's taunting speech - about forcing the issue.

Ironically, that dilemma may be taken out of Brown's hands on Thursday. For both Blair and Brown the most important scheduled event of the week takes place on the North Sea coast rather than the south coast. Thursday's Hartlepool byelection is the kind of contest that used to galvanise the whole of party politics and political journalism. Today, with byelections more ruthlessly controlled from the centre and hence less characterful, such contests are less high profile. But they still matter just as much as they used to.

It has been a long time since a party leader's future hung on the future of a byelection. But it is true of Blair and Hartlepool on Thursday. If the Liberal Democrat Jody Dunn manages to suck up the anti-Labour vote to defeat Labour's Iain Wright, then Blair's grip on power would be rocked. Blair has sometimes said that he would not hang around if he thought he was a liability to his party's electoral chances. Defeat for Labour on Thursday would leave him staring that conclusion in the face.

The more intriguing scenario, though, concerns the aftermath of a Labour victory. If Wright is elected to replace Peter Mandelson, then Labour will judge that the electoral sea-wall has held against the high tide of disaffection with the Blair government. A Wright victory will lead Labour to conclude that Blair is safe to lead the party into the general election.

It all depends on the scale of the Labour victory. Mandelson had a 38-point lead when he retained the seat in 2001. If Wright sees off the challengers with a win of that kind, then Blair is entitled to conclude that the worst is over. But if the lead is down to single figures, Labour MPs all around the country will be looking down the barrel of a gun. And that could start a process that even Gordon Brown's hard-wired caution may be unable to resist.

Guardian