Ming Campbell was once again forced to defend his leadership of the Liberal Democrats today – but ended up delivering an ill-conceived quip in front of the party faithful.

On the day that a Times poll showed that 67 per cent of Lid Dem supporters thought that he should be replaced with a "younger and more charismatic leader", Sir Menzies set out to charm delegates at the party conference in Brighton with a relaxed Q&A session hosted by the broadcaster Sandi Toksvig, a Lib Dem supporter.

Ms Toksvig was quick to bring up what she called "the elephant in the room" – the question as to whether, at 66, the Scot was not too for the job.

Sir Menzies said that he had learnt to be "thick-skinned". “It’s not so much about age. It’s about judgment and experience. It’s about Liberal Democrat values and doing your best to promote them," he added.

But, a few minutes later, he undid much of his own good work when he allowed the conversation to meander and recalled how Harold Macmillan, the late Conservative prime minister, would vomit in the corridor before crucial Commons appearances.

He then asked Toksvig if she too suffered from nerves before important engagements, which she admitted was the case, before adding: "Obviously you don't make me nervous. I'm all right."

"Well of course," Sir Menzies replied. "I'm a failure."

As self-deprecating one-liners go, it was inept and badly timed. Although the Lib Dem leadership insists that Sir Menzies's position is safe, the party is struggling at about 15 per cent in the opinion polls – down from 23 per cent at the last election – after being squeezed by both David Cameron's Tories and a resurgent Labour Party under Gordon Brown.

The party has also lost the electoral benefit it gained from its opposition to the Iraq war.

Nor can Sir Menzies expect any succour from his predecessor, Charles Kennedy, who was kicked out by the party early last year after admitting to a serious drink problem.

In an appearance on BBC Radio 4's World at One today, Mr Kennedy dismissed a suggestion that conditions were tougher now for the Lib Dems than they had been when he was leader.

“There’s nothing that has come as a surprise,” he said. “It is nothing that should be fazing us at the moment.”

Today's Populus poll brought bad news for the Lib Dems beyond the obvious negatives about Sir Menzies's leadership. The poll suggest that two-thirds of voters – including two-fifths of Lib Dem supporters, consider the party to be basically a protest vote party, decent people whose policies do not really add up. That perception is especially damaging if it leaves the party struggling to make a mark with voters.

In the meantime, the party is getting on with renewing its policy platform, delegates voting today to back ambitious proposals to make Britain carbon-neutral by 2050.

Steve Webb, the Lib Dem MP who chairs the manifesto working group, said today that he is to hand Sir Menzies a draft manifesto – "chock-full of bold, radical and distinctive policies" –

by the end of this week's conference, ready for a snap election.

Mr Webb is compiling that document through a series of one-to-one meeting with front-benchers, but has also called for input from another source – his 1,909 "friends" on the social networking site Facebook. A question on his profile reads: “If you could put one thing in the next Lib Dem manifesto, what would it be?”

In his conference appearance today, Sir Menzies said the speculation about an early election was a “great tease” by Gordon Brown that showed once again the need for fixed-term parliaments of four years.

He said that he did not think Mr Brown would seek an early poll, but added: “To have the mandate for those policies, I think it is necessary to go to the country.”

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