I remember that day well. I drove into town convinced that Clegg was about to do a deal with Labour and drove home to the news that a Con-Dem coalition was in the making.
Despite making a "generous offer" to Nick Clegg's party, he feared it would link up with Labour when the two parties opened rival talks. "On Sunday I was thinking I probably will be Prime Minister on Monday. I was thinking by the end of Monday I definitely won't be Prime Minister," he says.
When an impatient Gordon Brown resigned before the Tories and Liberal Democrats agreed a deal, Mr Cameron was taken by surprise. "I remember having to ring Samantha. She was doing Nancy's homework and I said, 'We could be going to the Palace – you'd better get your frock on'."
Of course, it now transpires that Clegg had no intention of ever forming a coalition with Labour and that he was merely going through the motions to please his own backbenchers.
But it's interesting that even Cameron appears to have believed that Labour were more natural bedfellows for the Liberal Democrats than his own party were.
It's a feeling which many of us shared.
Indeed, I think it's something which the Liberal Democrats might one day deeply regret. It's very hard to see what they are getting out of this deal. They are going to share the blame for Osborne's dreadful budget whilst the concessions they have achieved will possibly be utterly forgotten.
Sir Menzies Campbell, the former Liberal Democrat leader, warns that Mr Cameron and Mr Clegg might end up looking like each other, in the way that owners look like their dogs. "If you have a coalition partner then it seems to me there's a grave risk eventually you'll come to look like them," he says.Sir Menzies Campbell has got it in one.
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