There is a useful little mental test to determine a politician’s worth. If they did not exist, would it be necessary to invent them?

If you want to know why the knives are now being sharpened for Liberal Democrat leader Charles Kennedy, it is because there is absolutely no-one - least of all his fellow LibDem MPs - who knows what he stands for.

The truth is that the whole point of his leadership has become a mystery and if he did not exist, no-one would notice.

Of course, the LibDems' current crisis is much more deep-seated. Kennedy’s troubles are profound, but they have suddenly been highlighted by the emergence of another front rank politician.

David Cameron's election last week as Conservative Party leader has, at a stroke, changed the dynamics of British politics and signed Mr Kennedy’s political death warrant.

For, until now, most of 'Chat Show Charlie's' time as party leader (having taken over from Paddy Ashdown in August 1999) has been spent facing a moribund Conservative Party and an increasingly distrusted Labour government.

The Conservatives have been purposeless - and, in reality, leaderless - since 1990, when they removed Baroness Thatcher. Meanwhile, New Labour may have won a second election landslide in 2001, but that was only achieved because there was no credible opposition and because voters thought Tony Blair should be given a further opportunity to bring about the changes he promised in 1997.

By default, sections of the electorate turned to the LibDems as the lesser of three evils. This new-found support was enough to give the party 22 per cent of the vote in this year's General Election and saw 62 LibDem MPs returned to Westminster.

Buoyed by this artificially inflated backing, Kennedy has managed to portray himself as the personification of his party's membersship: a bunch of jolly, sensible, nice people who are above grimy, traditional two-party politics..

But that can never be a lasting strategy. It has only ever been possible in a political world where neither Labour nor the Conservatives have positive appeal. It merely relies on attracting voters on the basis of who the LibDems are not, rather than on the basis of their core values.

In the short term, Kennedy has successfully wooed disaffected Labour voters by positioning his party to the Left of Tony Blair (demanding tax hikes on the well-off and ever-increasing public spending). At the same time, he has wrested the support of former Conservatives who have become increasingly disenchanted with their own seemingly suicidal party.

But the emergence of David Cameron and a reinvigorated Conservative Party means that the game is up for Mr Kennedy’s strategy of being all things to all people.

The truth is now patently clear: many left-wing LibDem MPs are indistinguishable from left-wing Labour MPs, while many others at the tough, freemarketeer wing could easily join the Tories - with their new, more liberal, tone.

This latter group is represented by what are known as ‘Orange Book’
Liberals - nicknamed after a document published last year - who see freedom of the individual, competition and individual responsibility as the essence of liberalism. Like the Conservatives, they believe in markets, not the state.

More pertinently, Charles Kennedy is also about to be hit by another fundamental change in the political landscape.

The Labour Party is preparing to go undergo a major redefinition when Gordon Brown eventually takes over from Tony Blair.

However unpopular Mr Brown may turn out to be as Prime Minister, there will soon be a whole new political game, with Brown and Cameron setting out clear and contrasting agendas.

In the middle, with nothing distinctive to say and with nothing visionary on offer, the LibDems will be left exposed.

Gordon Brown’s appeal to disaffected Labour voters will soak up one side of the party’s voters; David Cameron’s revitalised Conservatives with its more socially liberal attitudes will entice the others.

It is this realisation which has suddenly dawned on those LibDem MPs who have made it clear that Kennedy's time is now up.

His lethargy ever since May's election - with barely nothing to say other than at Prime Ministers’ Questions - is the initial complaint made against him, but the real predicament goes far deeper.

For all Kennedy’s manifold flaws and for all the persistent rumours aout his drinking, his approach mirrors the fundamental problem at the heart of his party.

The LibDems have two irreconcilable wings. There are, as I have said, the Orange Book liberals. But the vast majority of party members are traditional ‘beard and sandals’ types, who are genuinely to the left of Labour. They are Eurofanatics, keen to sign up to the euro and to hand over ever-more power to the EU. They see America as the root of all evil. They want tax rates which would make Gordon Brown blush, and they think the answer to every problem is more spending.

If this agenda ever became the LibDems’ election manifesto, the mantle of ‘the longest suicide note in history’ would shift from Michael Foot's catastrophic 1983 Labour manifesto and, instead, it would be handed to the LibDems.

But even if Charles Kennedy is ditched as leader by his colleagues, this will not be the end of the party’s problems. Any new leader will find it completely impossible to lead a party that is split from back to front, from left to right and from side to centre.

Charles Kennedy has tried to be all things to all people - not just to the electorate but also within his own party. In doing so, he has never tried to resolve the fundamental difference of opinion between the opposing wings of his party.

In the past, when one of the two main parties was in decay, he could afford to ignore the split. But now, amid talk that he only has weeks left as leader, he must offer something positive if the LibDems are to keep their existing support, let alone make headway.

But Chatshow Charlie will find this beyond his abilities. Moreover, the longer he remains leader, the more he will look irrelevant alongside David Cameron and Gordon Brown.