Vote 2010: No time for Sam's birthday, says Cameron in Yorkshire

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The Tory leader did his best to look relaxed ahead of tonight's first live showdown with his main rivals, making pots of tea for parents and volunteers at a charity resource centre on a council estate near Halifax.

And he joked that he might dodge the high-pressure engagement by "accidentally" spilling the hot drinks down himself.

It was his only public appearance before tonight's unprecedented event - and Mr Cameron admitted he was nervous about fluffing his lines, despite insisting he was relishing the chance to debate in front of millions.

Aides said he would not be spending the remaining hours engaging in last-minute practice runs but would be studying his detailed notes about key issues and "clearing his head".

Arriving at the Mixenden parent resource centre, the semi-casually dressed Mr Cameron and his wife - who is expecting a baby in September - first sat down to paint with youngsters, with a disappointed Mr Cameron finding the blue pot already in use.

After noting there was a "Cameron" on the register, they moved on to the kitchen, where the would-be prime minister was put to work brewing up for the local people he was due to meet.

It was there he broke the news to Samantha that she may end up empty-handed on Sunday when she turns 39.

"I'm not going to have time to buy any presents," he quipped.

"But I'll do the cooking instead - will that be all right?"

Mrs Cameron - whom the Tory leader has dubbed his "secret weapon" - was campaigning alongside her husband for the first time since the election was called.

She wore a pink Tevrow and Chase blouse with a fuchsia cardigan and dark trousers, both from Jigsaw, and black Russell and Bromley shoes.

Mr Cameron - perhaps in a bid to hide his self-confessed nerves about the first debate - made a string of jokes, saying as he carried in the tray: "That would be one way of getting out of the debates if I pour the tea down myself."

And he declared he had made proper Yorkshire-style tea: "You could trot a small mammal across the top of that without it falling in."

Discussing the impending addition to the family, which he said was nicknamed "the bump", he said his two children did not seem to be missing him during his tour of the UK as much as he did them.

Daughter Nancy, six, had recently expressed herself "too busy" to speak to him on the phone before granting a brief audience, he revealed.

Mr Cameron said his main worries over the debates with Gordon Brown and Nick Clegg were "about not getting it right, about not getting the point across you want to make, those sorts of things.

"But above all, I am looking forward to it because it is a great opportunity. Normally you have to deal with the 30-second soundbite on the news and all politicians I think feel frustrated that we can't talk directly to the public about the problems the country has and what we would do to solve them."

He insisted it should not been seen as a fight, with opponents seeking to land knockout blows or exploit gaffes.

"The metaphors of boxing and bouts is wrong. If that's what it's about it will, I do not think, be the right thing.

"What it should be about is trying to explain how we get out of the hole we are in."