Sandwiches with a smile on the Tory battlebus

His two children Nancy and Elwyn keep asking him: "Why is the election taking so long?", But after two precious nights of good sleep on Friday and Saturday, it seems David Cameron would be quite happy if it lasted even longer.

His black folder tells him that yesterday is "D minus 4". In other words, he has just four precious days to convert the Tories' lead in the polls, which still point to a hung parliament, into an overall majority.

But Mr Cameron looks like a man with momentum. On a visit to Newquay in Cornwall and Holywell in North Wales, he is full of energy and charm – in his socks and black jeans, he serves sandwiches and cakes to his staff and accompanying journalists as his 25-seater private plane heads to the South-west. An embarrassed stewardess tries to stop him but he carries on down the aisle. "I have taken over," he smiles.

Momentum is the magic ingredient every politician craves in elections. So Mr Cameron is quick to convert his poll victory in last week's final TV debate into a claim that the Tories have won the argument on the economy. The mood in his inner circle is one of cautious optimism that they will cross the finishing line of 326 seats. But they know they need one final push to be sure.

In his interview with The Independent, Mr Cameron expresses delight that Liberal Democrat policies on tax and immigration are now under the spotlight. He suspects this scrutiny has stalled Nick Clegg's surprise advance. "They have been tested and you can see things falling apart. If we went into an election with a set of numbers on the economy like they [the Liberal Democrats] have, I would not be sitting here now," he told me on his battlebus. "I would be in deep, deep therapy having been completely destroyed. We would have been murdered."

A couple of years ago, Mr Cameron was asked for a political joke and replied: "Nick Clegg." Did he regret saying that now? Surely he had underestimated the Liberal Democrat leader?

It is the only question that produces a pregnant pause during the interview. "It was a joke," he said. "A good joke," he added. No regrets are allowed, even over the Tories' critical decision to allow Mr Clegg equal billing in the leaders' TV debates, which transformed the election and made Mr Cameron's path to No 10 much more rocky. "I always believed the TV debates would give an opportunity to the third party. They definitely did. They created a much more challenging election than might otherwise have been the case. I said five years ago that my TV debate with David Davis [during the Tory leadership election] should be a precursor for the general election. You can wriggle out of them like Tony Blair did but I am not that sort of person."

Mr Cameron dismissed as "complete rubbish" a claim by Vernon Bogdanor, his former Oxford tutor, that a Tory government would cut public spending disproportionately in areas where it is high. "He has got the wrong end of the stick," said Mr Cameron. "Even the most Trotskyite member of the Labour Party would accept we need a rebalancing where the private sector grows and there is some restraint in the size and scale of government. I think I will take some time out when this is all over and give him a tutorial of my own."