On Thursday a YouGov opinion poll for the Sun newspaper showed the Conservatives, led by David Cameron, holding a narrow lead with 33 per cent, the Liberal Democrats at 31 per cent and the Gordon Brown's Labour party at 27 per cent.

Media attacks

But the change in fortunes for Clegg's party, which normally come a distant third in general elections, has prompted scathing attacks from the country's right-wing press.

The tabloid Daily Express headlined "Clegg's Crazy Immigration Policy" while the Daily Mail led with "Clegg in Nazi Slur on Britain", referring to an article he wrote in 2002, which said Britain had "a misplaced sense of superiority" stemming from the defeat of Germany in World War Two.

In depth

A report in the Daily Telegraph accusing him of directly receiving funds from party donors was strongly denied by the Lib Dems.

"I must be the first politician who's gone from being Churchill to being a Nazi in under a week," Clegg said on Thursday.

"I hope people won't be bullied and be frightened into not choosing something different."

Peter Mandelson, a senior Labour politician, came to Clegg's defence, prompting new talk of a post-election pact between the two parties.

Expenses scandal

The accusations against Clegg come as British politicians are still struggling to repair their tarnished reputations after last year's expenses scandal, which revealed they had claimed public money to pay everything from adult films to tennis court repairs.

The Lib Dems had used the scandal to attack Labour and the Conservatives, insisting their own politicians had emerged largely unblemished from the affair.

Thursday's debate, which kicks off at 1900 GMT, will focus on foreign policy, including Europe, on which the Conservatives could be attacked over their past divisions on the issue, and the Lib Dems for wanting Britain to adopt the euro.

The wars in Iraq and Afghanistan will also likely feature highly in the debate, a thorny issue for Brown who has been accused of underfunding the armed forces, but also for the Conservatives, who strongly supported the unpopular conflicts.

On defence spending, the Lib Dems are the only party suggesting there should not be a like-for-like replacement of the Trident nuclear missile defence system.