Steve Richards: The first piece of good fortune for Gordon Brown

The extraordinary resignation of David Davis is a more dangerous moment for David Cameron than it seems. On the surface, there is unity. Mr Cameron has said he will campaign for his former shadow cabinet colleague in the bizarre by-election. The two of them have opposed, with forensic vigour, the Government's plans to extend the period for which suspects can be detained.

That is not, by any means, the full picture. If Mr Davis felt he had the full support of the Conservative leadership for his robust opposition, he could have taken his campaign to the country as shadow Home Secretary. There is no better platform to put the case.

But, in recent conversations with the Conservative leader, Mr Davis sensed some resistance.He did so with good cause. On several fronts, the Conservative leader adopts a bold tone when discussing general themes but tends to follow the most popular line, or more orthodox Tory solutions, in addressing specific policies. In this case, Mr Cameron evidently has worries about entering a general election campaign pledged to scrap a policy that polls suggest is popular.

Partly, Mr Davis wants to prove to Mr Cameron that his case can be made and won in an election or, to be more precise, at a by-election in his own seat. As he put it in an interview yesterday: "More people care about it than conventional politics realises."

Those words are highly significant. He did not state that more people care about this than the Government realises. By deploying the phrase "conventional politics" he was throwing a dart in the direction of Mr Cameron as well.

The situation is more complicated than simple electoral calculations. There are tensions in the Shadow Cabinet over the substance of the issue. When Gordon Brown first put forward his proposal last summer, Mr Cameron wavered. According to those close to Mr Davis, the two of them had several conversations before the Conservative leader decided to oppose the measure. An interview given by the shadow Security minister, Dame Pauline Neville Jones, on BBC's Newsnight this week highlighted a wider divide. She said a Conservative government would review its position in relation to 42 days but implied that such a review could lead to the Conservatives accepting the law rather than scrapping it. The Shadow Cabinet was genuinely united in opposing the muddled package presented by the Government this week, but some are against the muddle, not the principle of giving police more time to question suspects. With a genuine passion, Mr Davis is against the principle, as he was against the introduction of ID cards. On that issue, he fell out badly with the former leader, Michael Howard, who was a strong supporter of ID cards. They had fuming rows, some of them taking place on the telephone during the 2005 election campaign. Once more, Mr Davis displays a combination of high principle and egotistical showmanship. In doing so, he has lost the chance of being the next Conservative home secretary.

The moment is a big test for Mr Cameron because it could expose the deep divisions in the Tory party, not only over the relationship between civil liberties and security but on other big policy areas such as tax, Europe and the environment.

For more than a year, the media focus has understandably been on the traumas of the Labour Government. The Conservatives have escaped much scrutiny. That might change now. Mr Davis appears to be striking out against the policies of the Government, but partly he is expressing frustration with Mr Cameron. It is the first bit of luck Mr Brown has had since Labour's conference last autumn.