MENZIES CAMPBELL will today try to shore up his wobbling leadership of the Liberal Democrats by pledging to break the "cosy consensus" in which the other parties are failing voters.

In his conference speech in Brighton, Sir Menzies will condemn Labour and the Tories for failing on the main issues facing Britain.

Only the Lib Dems are prepared to offer voters a proper choice on the environment, inequality, pensions, nuclear power and Iraq, he will argue.

Sir Menzies will say: "What our country needs is a political party that's prepared to take the lead and speak the truth, because on so many of the major political issues there is a two-party consensus - comfortable, cosy and complacent."

Insisting British politics is "not two against one, but one against two", Sir Menzies will add: "That's a consensus that we alone can break, that we must break."

However, this morning's speech will be made against the damaging backdrop of sniping about Sir Menzies' leadership breaking out into the open in Brighton.

His aides were left cursing Nick Clegg, the ambitious home affairs spokesman, who let slip that he would run for the leadership "if there was a vacancy".

The comment was quickly criticised by environment spokesman Chris Huhne, Mr Clegg's most likely rival to succeed Sir Menzies, as "premature".

Mr Clegg later defended his speculation about the succession, saying: "I'm afraid I'm not the kind of person who dances on the head of a pin and ducks questions."

The 66-year-old leader was forced to insist he was "totally relaxed" about the jockeying, saying: "I like ambition. I have been ambitious myself all my life."

But Sir Menzies reminded all young hopefuls: "Let me make it very, very clear - there is no vacancy. I shall lead this party through this parliament, through the general election and into the parliament beyond."

Later, in a chance encounter outside the conference centre, Sir Menzies' wife, Elspeth, was overheard saying to Mr Clegg: "I don't know if you're being helpful or not."

He replied: "I'm trying to be."

Today, the leader will attempt to convince delegates worried by his inability to shake off questions about his age that the Lib Dems are poised to exploit a "huge opportunity".

His aides told journalists that Gordon Brown's determination to stick to his "Blairite legacy" meant both Labour and the Conservatives were now camped on the "same narrow battleground".

In contrast, the Lib Dems had costed, thought-through policies - to tax pollution, not work; to create a low-carbon energy policy; to scrap tuition fees; and to set a timetable for withdrawal from Iraq.

Sir Menzies will say: "Only we can achieve that free, fair and green society, because only we believe in it. Only we will work for it, only we will fight for it."

He will add: "Today, our party is not only the real alternative, it is the only alternative."

The Lib Dems were buoyed by an eve-of-speech poll that put the party up two points on 20 per cent - far better than recent surveys that recorded a score of 15 per cent.