Danny Finkelstein has a theory:

The Macavity theory is that fighting an election you are bound to lose takes a good deal of courage. It requires you to show up every day and take personal charge.

Wouldn’t it be exactly in Mr Brown’s character, goes the theory, to disappear just as the election was about to start, when loss was inevitable?

Well… no. Because even though he is Macavity the Mystery Cat, he is Gordon Brown – and he will never accept that he has done wrong or that it is inevitable that he will lose. He simply will never accept it.

Browns major flaw isn’t that he is Macavity, it’s that he can’t accept that he has ever done anything wrong.