Alarmed Conservative and Labour strategists are planning an unprecedented attack on the Liberal Democrats after Nick Clegg's victory in the first leaders' debate transformed the general election into a three-horse race.
According to a poll published today, the Liberal Democrats leapt into second place after the programme, suggesting that the party was well-placed to make a dramatic breakthrough in the closest election contest for nearly 20 years.
The YouGov survey in The Sun puts the Tories on 33 per cent, the Liberal Democrats on 30 per cent and Labour on 28 per cent. Repeated on an even swing at the election, those figures would produce a hung parliament with Labour the largest single party with 276 MPs, against 246 Tories and 99 Liberal Democrats.
It echoed an earlier poll that discovered a surge in support for the party after Mr Clegg went head to head with David Cameron and Gordon Brown.
The historic 90-minute encounter between the leaders attracted up to 10.4 million viewers. As Mr Cameron and Mr Brown acknowledged that Mr Clegg had performed strongly, their parties were poised to turn the spotlight on Liberal Democrat policy on tax, spending, immigration and Europe.
The Tories, in particular, are expected to go for the jugular amid fears in Conservative headquarters that a strong Liberal Democrat showing could pave the way for a deal between Mr Clegg and Mr Brown in a hung parliament.
Mr Clegg will spend today preparing for next week's debate and steadying himself for a sustained attack from his rivals attempting to reverse his electoral bounce.
In an interview with The Independent last night, he admitted that he had been "terrified" at being the first leader to speak in the debate. "You would have to have a heart of stone not to be nervous. But after a while, I genuinely started to enjoy it. When they said 'That's it,' I thought they meant it was the end of the first half."
Speaking on a train to London after campaigning in Hull, he said: "I have been mocked in the past for saying I want to be Prime Minister – I'm not going to apologise for my ambition."
Mr Clegg insisted he was not intimidated by the threat of attack from the Conservatives. "If the Tories want to have a contest of ideas instead of a contest of advertising budgets, I am very confident we can win that. The Tory strategy has been about avoiding talking about policy, and spending money to win the election. If we can pull them off that strategy, it will have been a huge success."
Next week's leaders' debate will focus on foreign affairs. But Mr Clegg made clear he would not spring any surprises over the conflict in Afghanistan, such as backing a phased withdrawal of troops.
The Tories' opening shots against the Liberal Democrats were fired by Michael Gove, the shadow Children's Secretary, who highlighted their plans to join the single currency, scrap the Trident nuclear deterrent and give an amnesty to illegal immigrants.
Mr Gove said: "The greater degree of scrutiny these policies have, the more people will realise that, while Nick Clegg is a very attractive individual in many ways, the policies of his party are outside the mainstream and a little bit eccentric – not necessarily what you would want at a time of crisis."
Meanwhile, Douglas Alexander, Labour's election co-ordinator, said: "Mr Clegg was always bound to gain from equal billing and he presented his case well. But his policies have yet to come under real scrutiny. People will be in for some surprises about the content of Liberal Democrat policies."
Labour's focus will be on the party's plans to save £1.3bn a year on tax credits by restricting them to the worse off, restrict winter fuel payments to the over-65s, and end contributions to child trust funds.
Tory strategists fear a Liberal Democrat advance could deprive them of target seats held by Mr Clegg's party in the South-west of England. Labour will be happy for Liberal Democrat support to firm up against the Tories, but they will be worried that a surge in backing for the party could cause a stream of Labour casualties in the north of England.
The potential for the debates to give Mr Clegg a new political platform was also underlined by the ComRes poll conducted among 4,000 people who watched the debate. It suggested that the Liberal Democrat leader's performance lifted his party's support among them to 35 per cent, up 14 points from before the programme, while support for the Conservatives dropped three points to 36 per cent and for Labour by three points to just 24 per cent. Extrapolated across the country by taking into account non-viewers, the Tories would be on 35 per cent, Labour on 28 per cent and the Liberal Democrats on 24 per cent.