By Nick Assinder Political correspondent, BBC News website

So Charles Kennedy has survived what was almost certainly the most difficult day for his leadership since he took over from Paddy Ashdown more than six years ago

Mr Kennedy's leadership has provoked questioning

He has even attempted to put a positive gloss on the events of the past 24 hours or so, suggesting all the complaints about his leadership style were evidence of a party thirsting for power.

To an extent that is true. The most optimistic Lib Dems believe they have made huge advances in recent years and may really be on the way to, at worst, genuinely becoming the official opposition and, at best, the government.

But what keeps them awake at night is the nightmare thought that it may simply all melt away like snow on a ditch, as it has done before.

While Mr Kennedy led his party to one of its best election performances ever, there have been persistent worries that, if the Lib Dems were to make the longed-for breakthrough at the next general election, they really should have done better last time around.

'One-of-us air'

And ever since the last election there have been rumblings about Mr Kennedy's style and whether he is the man to lead the party to that crucial next poll.

Now, to make matters worse, his MPs have taken a look at new Tory leader David Cameron - and got a double dose of the jitters.

Behind all this is the thought that it is far too early to panic in the face of David Cameron Who might succeed Kennedy?

They see a young, modern, dynamic leader whose consensual approach to politics echoes much of Mr Kennedy's style, but with added oomph.

Meanwhile, they also may be facing a new, formidable and determined Labour prime minister in the likely shape of Gordon Brown at the next election.

Senior party members fear that even a moderately revived Tory party under Mr Cameron will see them seizing back local council and Westminster seats, while their biggest anti-Labour platform - their opposition to the Iraq war - will be denied them next time.

What is needed, they argue, is a dynamic, charismatic and healthy leader to up the party's game.

But a number of things appear to have worked in Mr Kennedy's favour and headed off an immediate challenge.

Firstly, whatever his Westminster critics believe, he is a popular politician who has a natural, unaffected, one-of-us air which voters like.

And what none of his critics know for sure is exactly what proportion of their vote is a direct result of Mr Kennedy's personal qualities. Secondly, his detractors do not seem to have got their act together.

Mr Kennedy has been Lib Dem leader since 1999

If they were planning a coup they didn't organise it very well and it appears to have collapsed at the first sign of a challenge - perhaps they were expecting him to simply cave in.

Thirdly there is the difficult question of who would replace him. His deputy, Menzies Campbell may be keeping his head down, raising speculation he wants the job, but he will be close to 70 at the next election and has had health problems in the past.

Some Lib Dems worry about whether he really is the sort of character, impressive though he is, to challenge Mr Cameron.

'Front foot'

Then there is Simon Hughes whose leadership ambitions are well known, but who commands limited support in the parliamentary party. Or could it be one of the younger MPs like Mark Oaten or even David Laws who remain unknown quantities.

And behind all this is the thought that it is far too early to panic in the face of David Cameron who is himself un-tested. Perhaps he will go the way of four Tory leaders before him.

None of this is to suggest that Charles Kennedy is free and clear, however. He has promised to listen to his critics and return from the Christmas break "on the front foot".

He probably has months rather than weeks to do just that and if he succeeds there is no reason to think he will not take the party through to the next general election.

If he appears to fail then the next move against him will almost certainly prove fatal.