To accuse someone of crawling is horrible. There is something so abject, so self-abasing, so lacking even the pretence of dignity in the word “crawl”. David Cameron today accused Ed Miliband of “just trying to crawl through the gates of Downing Street on the coat tails of the SNP”.

The Prime Minister used variants of this phrase at least four or five times. On one occasion the crawling took place “in Alex Salmond’s pocket”, rather than on his coat tails, but the meaning of the phrase was just as contemptuous. A moment later, Cameron called Miliband “weak and despicable”, as if not certain whether the point had been made clearly enough.

And yet a few minutes after that, the Prime Minister produced a graceful and affectionate tribute to David Blunkett, one of the great Labour figures of recent times, who will be retiring from the House at this election. And whenever he is addressed by a woman on the Opposition benches, Cameron takes care to respond to her with courteous decency.

Two different men appear to inhabit the Prime Minister’s body. One is generous, affectionate, always ready to see the best in his fellow man or woman. The other is mean, hateful, always ready to see the worst in the Leader of the Opposition.

What a pity Robert Louis Stevenson is unavailable to write about this phenomenon. In Dr Jekyll and Mr Hyde, he produced a brilliant study of a man who discovers how to experience such duality of spirit. By chemical means, the delightful Dr Jekyll transforms himself into the evil Mr Hyde, with effects that are in the end disastrously self-destructive. The Prime Minister seems, by contrast, to achieve the same effect simply by contemplating the imminence of the general election, and we do not yet know how his story will end.

But one hopes that during the general election campaign, we shall see the generous and affectionate Cameron, always ready to see the best in other people. For the other Cameron is not a person anyone not already deeply loyal to him would be likely to choose to vote for.