John Rentoul: Charles Kennedy's fate is sealed. The only question is who will wield the knife

The theme is persistent: that Kennedy is not 'active' enough

It was one of those head-in-hands moments that define political careers. Two weeks ago, Charles Kennedy was asked on television whether any of his MPs had told him, for the sake of his party, that he should go. This is what he said: "Nobody was looking at all, in terms of the contributions in our private discussions as well as in the course of the wider parliamentary meetings that I've had, at any need or sense whatsoever to start at this stage - the week after a new Conservative leader - the process of a Liberal Democrat leadership election."

It was Jim Mortimer, Neil Kinnock and Iain Duncan Smith rolled into one. (Mortimer was Labour's general secretary, who told an astonished press conference during the 1983 election campaign: "The unanimous view of the Campaign Committee is that Michael Foot is the leader of the party.")

It would be reading too much into a single media performance to say that Kennedy's weird interview sealed his fate. It did seal his fate, but not just because he sounded like a dead man talking. Kinnock gave plenty of embarrassing performances - but he could turn in some terrific speeches too. Above all, he could fight when his back was against the wall. Kennedy is capable of neither.

It was often said of Kinnock that he was protected by the cumbersome procedures for Labour leadership elections. The comparison was made with the apparent ease with which Michael Heseltine had been able to strike against Margaret Thatcher in 1990. All Heseltine needed was a proposer and a seconder to trigger a vote of Conservative MPs. Labour's rules required, and still require, a vote at annual party conference to start a leadership election.

However, the politics is more important than the rules. That has been true for potential assassins from Heseltine to Sir Menzies Campbell. The reason why Heseltine's challenge unseated Thatcher, although it did not yield him the crown, was because most Tory MPs thought that she would lose them the next election. The reason John Smith never struck against Kinnock before the 1992 election is that Kinnock was generally expected to win it, even after Thatcher was replaced by John Major.

Similarly, Michael Portillo had the phone lines installed but never used them because he realised that he could not beat Major in 1995. Brown has the phone lines, the staff, the first-hundred-days plan and the power base at the Treasury - and yet he holds back. This is not because he lacks the nerve to strike, as has often been alleged, mostly by those on the left who wish he would. It is because he rightly judges that he cannot be sure of securing a vote of no confidence in Tony Blair among Labour MPs, which is the only way of short-circuiting the formal rules. That is because it is not yet - as it has not been for eight and a half years - sufficiently in their interest to ditch Blair for Brown.

Where, then, does this leave Charles Kennedy - leader of a party that is surprisingly likely to hold the balance of power in a hung Parliament next time? His position is certainly as secure as it can be made by fixing the party rules. It was not widely noticed in June, just after the general election, that he was re-elected unopposed to the Liberal Democrat leadership. The rules provide for only one leadership election between general elections. As far as the rules are concerned, therefore, that is the end of the matter. But the political situation is not so cut and dried.

His party and his MPs are - a little unfairly - dissatisfied with the outcome of the general election, even though it returned 62 Liberal Democrats to Westminster, a higher total for the third party than at any time since 1923. This falling short of inflated expectations is linked - again a little unfairly - to criticisms of Kennedy's leadership. The theme is persistent: that he is not "active" enough. It is a conveniently vague criticism. Unconsciously, it invokes the memory of Paddy Ashdown, Kennedy's yomping predecessor, about to stand down this month as High Representative for Bosnia and Herzegovina.

In fact, when Ashdown resigned as Lib Dem leader in 1999, many in his party were relieved - his ceaseless activism led him to take many positions that they found difficult. He was still trying to form a coalition government with Blair, and took a muscular interventionist line on foreign affairs. Had he remained in the House of Commons, he might even have supported the invasion of Iraq. It is perfectly possible that Kennedy's passive leadership allowed the party to gain more votes than under a more "active" alternative.

But the party faces a different situation now. Iraq will not be an issue at the next election. The Liberal Democrats need a period of activism, almost for the sake of it, to hold on to any of the centre ground invaded so boldly by David Cameron.

The political situation has moved decisively against Kennedy. There was another little-noticed event last year that makes his position less secure. The party conference in September agreed a change to the leadership election rules known as the "Simon Hughes amendment", after the Lib Dem MP of the same name. In the event of a contest, MPs now must be nominated by 10 per cent of their colleagues to put themselves before the grassroots members, who have the final say. Although Kennedy presumably accepted the change on the assumption that it would make a challenge to his position more difficult, it has had the opposite effect. It removes the danger that Simon Hughes, who is popular with the membership but less so with his fellow MPs, would win, because he would be unlikely to gain the seven nominations required to stand.

If, therefore, a majority of Liberal Democrat MPs forces a leadership contest by writing to Andrew Stunell, the chief whip, to say they have no confidence in Kennedy, they have a safe default option in Sir Menzies Campbell, the deputy leader and foreign affairs spokesman. It would involve skipping a generation - Sir Menzies is 64 - but he is more likely to be a caretaker leader while an election is held.

No wonder Sir Menzies was so silky in his defence of his boss. "I want him to stay as leader, operating at the top level of the full range of his abilities," he told the Today programme at the height of last month's crisis. It is precisely that mix of ambition and media fluency that guarantees Sir Menzies would never be a tongue-tied embarrassment to his party. But he is more likely to be a transitional leader to someone nearly unknown, namely Nick Clegg. Kennedy will be lucky to make it through this month, let alone this year.