David Davis vs Miss Whiplash

It also appears, as Mr Cameron admitted in the Commons on Wednesday, that the public back many of Labour's measures. The Tory line is that they are doing what is right, not what is popular.

When news first broke this afternoon that Mr Davis was resigning, there was shock and confusion at Westminster. Nobody had expected it, and the best guess of most commentators was that he had fallen out in some way with David Cameron.

The explanation, when it came, was very different, but the suspicion remains that Mr Cameron is very unhappy with what has happened. He immediately appointed a new Shadow Home Secretary, making it unlikely Mr Davis will get his old job back if he is returned to the Commons, and wished his colleague luck in "his" by-election campaign.

Admittedly, Mr Cameron also said that he would head to Haltemprice and Howden to support Mr Davis at some point. But this whole thing is an unnecessary distraction when the Conservatives appear on course to win the next election - as long as they manage not to blow it.

The Tory leader certainly hasn't encouraged Mr Davis to do this. As one Shadow Cabinet member said: "He has walked the plank without anybody pushing him". And Conservative opposition to Labour policies such as ID cards and 42 days detention for terror suspects is already clear. They have been getting their message across without needing to indulge in stunts.

So what's in it for Mr Davis? He stood against Mr Cameron for the Conservative leadership and lost, and one theory among fellow journalists is that he is trying to position himself as a possible contender again. I don't agree. If the Tories win an election, which looks like a real possibility, Mr Cameron will be going nowhere except into Number Ten. Mr Davis would not give up the chance to be Home Secretary on the vague hope that Gordon Brown will stage a comeback and humiliate Mr Cameron at the polls.

Perhaps the most important question, and one nobody can answer yet, is what the public will make of it. Perhaps voters will admire Mr Davis for taking a brave stand on a matter of principle. But Labour MPs are delighted. They believe that, for a change, it is the opposition party that has scored an own goal.