Sir Menzies Campbell was struggling to reassert his authority last night after an attack by one of the party’s founding fathers and a new poll showing that two thirds of voters believe that the Liberal Democrats are the “protest vote” party.

The Liberal Democrat leader also managed to upset some of his MPs yesterday by saying that the top earners had done too well and agreeing that his tax plans would “hammer” the top 10 per cent. One MP involved in discussions on the policy suggested that his remarks, on the opening morning of the annual conference in Brighton, had gone too far and went beyond the agreed message on fairness.

A Populus poll for The Times, taken last week, spelt new trouble for the Lib Dem leader, with 67 per cent of party supporters saying that Sir Menzies should be replaced with a younger and more charismatic leader. This contrasts with members of the parliamentary party, who believe that it would be unwise to change leaders at this point.

The poll underlines the problem Sir Menzies has in making a public impact when both main parties have had new leaders, both much younger than him. David Laws, the schools spokesman, was forced to deny that the party had become little more than a think-tank for Labour and the Tories to cherry-pick ideas from.

The Populus poll suggests that two thirds of voters, including two fifths of Lib Dem supporters, think that they are basically a protest vote party and that, although decent people, their policies probably don’t really add up.

Sir Menzies defended his leadership yesterday, insisting that he had successfully maintained its distinctive position as the real opposition in British politics. “I have made it clear I will lead the party through this Parliament, into the general election, and into the Parliament beyond,” he told the BBC One Sunday AM programme.

He admitted that he was preparing the ground for an eventual leadership election, saying that he had promoted some younger figures to give them a chance to show off their talent before such a contest.

Sir Menzies suffered fresh embarrassment yesterday from members of the Gang of Four, the Labour politicians who broke away in 1981 to found the Social Democratic Party, which later merged with the Liberal Party to become the Liberal Democrats. Lord Owen, the former SDP leader, disclosed that he had been to see Gordon Brown last Wednesday. Lord Owen, a crossbench peer, said that he might back Mr Brown at the next election.

Lord Rogers of Quarry Bank, another member of the Gang of Four, attacked Sir Menzies’s leadership. “There is a feeling that there is a lack of energy and an absence of direction, so I very much hope the conference will be different and change the mood really,” he told the BBC One Politics Show. “It is easy to say that [the party] is more of a think-tank perhaps than a political party . . . you have got to be going straight to the central issues that matter to people. It is not happening.”

Linda Jack, a member of the party’s policy committee, joined the attack last night when she said that Sir Menzies had been “over-promoted”. She said: “Someone can be a brilliant man and have incredible intellectual powers and all the rest of it but if that does not translate into leadership skills then whoever your leader is you have got a problem with them.”

Of the other two members of the Gang of Four, Baroness Williams of Crosby has come under fire for agreeing to be an adviser to the Prime Minister. Roy Jenkins, the fourth member, died in January 2003.

Today’s agenda

9.30am Speech by Vince Cable, Treasury spokesman 10am Debate on climate change 12pm Steve Webb, manifesto chief 2.50pm José Manuel Barroso, European Commission President 3.15pm Sir Menzies Cambell Q&A 4.30pm Danny Alexander, Work and Pensions spokesman, and Julia Goldsworthy, Treasury

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