Analysis By Brian Wheeler BBC News political reporter

As Charles Kennedy reflects on what has probably been the worst week of his political life, he will not have to look too hard for the cause of his misery.

Mr Cameron's arrival has thrown the Lib Dems into near panic

And, unfortunately for the Liberal Democrat leader, there is not a great deal he can do about it.

Of course, Mr Kennedy was facing leadership problems well before David Cameron arrived on the scene.

This year's Lib Dem conference - billed as a "celebration" of the party's election gains - was overshadowed by mutterings about a possible challenge.

But the coronation of the telegenic new Conservative leader - and his subsequent land grab of traditional Lib Dem territory, on the environment and social justice, has raised those mutterings to fever pitch.

Not one of Mr Kennedy's potential challengers has been prepared to declare their hand yet - but their public protestations of support have each come with a sting in the tail.

Cunning

Sir Menzies Campbell and Mark Oaten, to name but two, say they will not topple Mr Kennedy, but they may be ready to run if he stands down.

Without Mr Kennedy at the helm, the party would almost certainly lurch to the right or left

Sir Menzies has said Mr Kennedy, like the rest of the party, will have to perform much better if they are to effectively challenge Mr Cameron for the increasingly crowded centre ground.

"It's not surprising that we should have to raise our game because the political landscape has changed with the advent of David Cameron," he told BBC Radio 4.

And - to make matters worse for the Lib Dem leader - Mr Cameron, perhaps scenting his first scalp, is now deliberately targeting Mr Kennedy (while appearing, for different but no less cunning reasons, to back Tony Blair all the way).

He has not mentioned the Lib Dem leader by name, that would be too much like the "yah boo" politics he professes to detest.

But, in a speech on Friday, Mr Cameron made a direct appeal to Lib Dem MPs and councillors to jump ship and join the Conservatives.

Iraq war

He claimed the Conservatives now stood for "liberal values", including a commitment to green policies and localism.

And, he added, they even share the Lib Dems' policy on Iraq, as they both wanted British troops out as soon as possible.

The Conservatives have even set up a website libdems4cameron aimed at recruiting disaffected Lib Dems to their cause.

The average Lib Dem supporter may find Mr Cameron's support for the Iraq war at the time of the invasion and his confirmed Euroscepticism hard to swallow.

And while the new Tory leader has talked a good fight on climate change and social justice, he will not be producing policies in these areas for 18 months.

But the message is clear - come on in the water's fine.

Vulnerable

Behind these warm overtures lies cold electoral calculation - and this, you suspect, is the real reason the Lib Dems have been thrown into near panic by Mr Cameron's arrival.

Despite electing 62 MPs at the general election, their highest number since the 1920s, the Lib Dems are highly vulnerable to a Conservative resurgance in several parts of the country.

The Lib Dems gained 12 seats from Labour, with no losses, but against the Conservatives, they won three and lost five.

Overall, according to BBC analysis and research, the Lib Dems' worst performance came in the seats they were defending where they faced a Conservative challenger.

In other words, it would take only a relatively modest surge in Conservative support to start unseating Lib Dem MPs.

Change

In his speech, Mr Cameron said: "If there was an election tomorrow, the Conservative Party would need to win 126 seats to win an overall majority.

"Of those seats, 20 are held by the Liberal Democrats, with the Conservatives in second place in every single one.

"And in all but four of the rest - that is over 100 constituencies - the Liberal Democrat vote is larger than the Labour majority.

"So I believe there is a new home for Liberal Democrat voters - and so a real prospect of a change of government."

'Tough liberalism'

Also, as Mr Cameron will know only too well, while the majority of Lib Dem members are traditional, left-leaning liberals, many of the party's brightest frontbenchers are much more right wing in their instincts.

Figures such as Mark Oaten, with his "tough liberalism", and David Laws, co-author of the Orange Book, advocating less regulation and greater use of the free market in public services, would not sound out of place in Mr Cameron's "Compassionate Conservative" party.

Simon Hughes, on the other hand, is firmly identified with the left of the party, and is arguably more in tune with the grassroots membership, as demonstrated by his election last year as party president.

Mr Kennedy's great skill as a leader has been to keep a lid on this tension - until now - while pulling off the difficult task of appealing to both Labour and Conservative voters at the ballot box.

Without Mr Kennedy at the helm, the party would almost certainly lurch to the right or left.

He will no doubt be hoping Mr Cameron's audacious tactics backfire and lead Lib Dem MPs to rally round him.

Otherwise the past week's internecine warfare will start to look like a minor spat between friends.