• The platform for tonight's debate
In a poll published today, six out of ten people say they intend to tune in to the three debates, the first of which will be screened this evening on ITV. Of those, a half – equivalent to 15 million people – say the debates will have some influence over their vote come 6 May.
The ComRes poll, commissioned by ITV/the Independent, offers a dramatic illustration of the power that the debates are set to have over the election result, heightened by the fact that millions of voters are still undecided about the choice on offer.
Last night, the Conservatives were actively trying to play down expectations of Mr Cameron amid fears within his party that his status as election front-runner has set him up for a fall on live TV.
Party sources said that Mr Cameron had not had enough time to prepare as he would have liked. However, it emerged that the Tories had won the coveted central podium for their leader, thereby enabling him to appear more "presidential" this evening.
• How the party leaders will perform in tonight's TV debate
Ahead in the polls, any slip-up, gaffe or unconvincing performance could damage his prospects. "Cameron has to play safe, he has everything to lose," former Downing Street spin doctor Lance Price said.
The pressure on Mr Cameron, and his two fellow leaders, was cranked up further last night as it emerged that ITV is to release a poll on who "wins" the debate within minutes of it finishing.
ITV declined to speculate on how many viewers it was expecting this evening, but the ComRes poll of 1,000 people found that only 31 per cent said they would not be watching any of the debates. Half of those who said they would be watching agreed that they were "open" to the debates influencing their vote.
With the stakes so high, all three party leaders will today carry on with intense preparations for the first debate, in Manchester, to be screened at 8:30pm.
Labour and Tory sources have both declared their intention to ensure Mr Clegg does not get an easy ride. Both the parties feel that the Lib Dems received such an electoral gift a during the chancellors' debate two weeks ago.
• Profile: Alastair Stewart, chairman of the leaders TV debate
However, it is Mr Cameron who knows he has most to lose tonight, as he protects his long-held lead over Labour in the polls.
Yesterday, he admitted he was "nervous" about the debate. A Conservative spokeswoman added: "It would be fair to say that it has been playing on his mind somewhat. He has not had as much time to prepare as he would have liked because he has been spending so much time going up and down the country."
Mr Cameron has previously expressed concern that the tightly drawn rules – the result of tortuous negotiations between the parties and the broadcasters – could produce an encounter that is "a bit slow and sluggish".
The Tory leader is widely seen as more comfortable in front of the television cameras than Mr Brown, but Conservative aides were keen to play down suggestions that he was the automatic favourite to come out on top.
One senior source suggested that Labour was also pursuing an "underdog strategy", deliberately damping down expectations as to how Mr Brown would perform.
Labour, meanwhile, is hoping that the debates can give Mr Brown a fresh chance to haul in the Tory lead.
• Thrashing out format for historic TV leaders' debate
Lord Mandelson said yesterday he had advised the Prime Minister: "Be yourself, be clear, be straight with people. Don't compete with Mr Cameron's personal insults."
The Liberals are hoping that the debates will give them a major boost, with the leader given equal status with his two rivals. Mr Clegg is understood to have been reserved the centre spot for the second debate, to be broadcast next week.
All three leaders have been rehearsing for the event, using colleagues to play the parts of their political rivals.
Tony Blair's former press secretary Alistair Campbell has reportedly been playing the role of Mr Cameron for Mr Brown, while Theo Bertram, a former No10 adviser, has been taking the part of Mr Clegg.
For Mr Cameron, shadow immigration minister Damian Green has taken on the role of Mr Brown while shadow culture secretary Jeremy Hunt has been acting as Mr Clegg.
The Lib Dem leader has used home affairs spokesman Chris Huhne to act as Mr Brown, and children's spokesman David Laws to play Mr Cameron.
Meanwhile, First Minister Alex Salmond has continued his protests over his exclusion from the debate.
The SNP says the broadcasters are breaking communication guidelines on impartiality by allowing the debates to be broadcast without them.
As well as an immediate poll after the debate, ITV is planning to screen a poll "worm" which will show the peaks and troughs of support for each man.
In addition, as the debate is on-going, thousands of people are expected to rate the leaders on-line using social media sites.
Facebook users will be able to link to a special "dial", so they can indicate in real time how strongly they agree of disagree with what they are hearing. Last night, nearly 40,000 people had signed up.
Richard Allan, Facebook's director of European public policy, said: "The Facebook dial test enables millions of voters to say exactly what they think of the three leaders' performances."
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