It is a fascinating political stunt, and you have to give credit to Davis for his understanding of political drama - an understanding lamentably absent when he fought for the leadership. But we should understand first, that this is a wholly unnecessary gesture, and second that it is not much of a gamble.
First, David Davis has always opposed the 42 days detention bill, and the other encroachments on civil liberties that he is fighting this by-election about. His position has not changed. His constituents know that their MP opposes further attacks on civil liberties, as he sees it, in the form of ID cards, 42 days detention etc. He is standing for re-election on exactly the same platform that he is currently representing his constituents. There is no change in his position. He is still a Tory. He has represented his views in parliament, for his constituents, with consistency. There is therefore no reason whatsoever for him to stand down and fight a by-election. It is an utter waste of public funds, and it asks for no new or fresh support for a radically changed position.
Furthermore, it is difficult to argue, a mere few weeks after Labour's catastrophic defeat in Crewe and Nantwich, that this is a big gamble. If Labour can't win in Crewe, the chances of them taking Davis' 5,000 plus Tory majority away from him in Haltemprice is as likely as Tony Blair apologising for something he was actually responsible for. Davis has taken a shrewd look at the political weather, seen that he risks very little for his dramatic gesture, and stepped into the political limelight with considerable agility. The main risk to him is that he becomes seen as irrelevant, or trivial.
And what of the Tory party? Cameron can hardly be pleased about this unnecessary political distraction. The last thing he needs is a prima donna prancing around the corpse of Gordon Brown. Once Davis is re-elected, there is little Cameron can do to avoid re-appointing him to his old job, with added grassroots support for admiring Tory members. Which is a pity, as the one good thing to come out of this is Dominic Grieve's well deserved shadow cabinet promotion as Davis' successor. Should Davis falter - and the political landscape is nothing if not rock-strewn - then the Tories will at least benefit from Grieve's new prominence.