Mr Cameron wants a united anti-Labour opposition

In a speech he said Conservatives stood for "liberal values", including a commitment to green policies, localism and deregulation.

Mr Cameron called for the creation of "a modern, progressive, liberal, mainstream opposition to Labour".

But Lib Dem leader Charles Kennedy said his rival should change parties if "he meant what he was saying".

'Whiff of opportunism'

Mr Kennedy added: "The one thing he is undoubtedly correct about in his speech is that an ever increasing number of people in this country want Liberal Democrat values and policies.

"In the new era of three party politics, the way to get them is to vote for the Liberal Democrats.

As long as Charles Kennedy remains leader of the Liberal Democrats, he has my full support Sir Menzies Campbell Lib Dem deputy leader Analysis: Cameron scents blood Who's who: Potential leaders

"If David Cameron actually meant what he was saying, perhaps he's the one who should be joining the Liberal Democrats and the Conservatives should be looking for a new leader.

"But this has the whiff of spin over substance - and given the timing - of opportunism."

Mr Cameron's comments follow a difficult week for Mr Kennedy, during which senior party members have criticised his style as too "laid back".

'Same side now'

In his speech in Lib Dem-held Hereford, he said the Conservative Party was a "new home" for Lib Dems and that both should combine to beat Labour in marginal seats.

He argued that Lib Dems and Tories were "on the same side now" over the Iraq war, as both supported "democracy" and "security" there and "our troops coming home as soon as possible".

HAVE YOUR SAY Kennedy is the symbol of the Lib Dems only shooting for second place Adam Drummond, Leicester Send us your comments

Mr Kennedy asked: "Is he really saying that his party - which supported the Iraq war - was wrong?"

Earlier, Lib Dem deputy leader Sir Menzies Campbell said his party and its leaders must "raise their game" to cope with Mr Cameron, adding that he would not be standing against Mr Kennedy.

Home affairs spokesman Mark Oaten also said he would not challenge Mr Kennedy but would run for leader if he stood down.

Mr Kennedy has promised the Lib Dems will come back united - and combative - in the New Year.