There's a lot of risk in it

His resignation as shadow home secretary, and decision to stand down as an MP, forcing a single-issue election in his Yorkshire constituency, are the culmination of tensions in the shadow cabinet about the vote on 42 days' detention.

Specifically, they reflect arguments around Mr Cameron's reluctance to commit himself to repealing the decision if he wins the next election.

Ardently as the Tories opposed this Bill, and furious as many were that such legislation managed to be passed only by virtue of votes cast by the Democratic Unionists and an apostate Ann Widdecombe, Mr Cameron has always been mindful that the Government has public support on tougher measures to combat terror.

Neither does he want to be boxed into an endless quest on the matter.

The strong libertarian strain in Mr Davis's speech - which implies that his battle to repeal 42 days will spread quickly into a wider crusade against the power of the state on issues like CCTV cameras - will also wake anxieties.

These things have a habit of spreading into wider rifts - remember that Mr Davis opposed Mr Cameron for the leadership. The outcome is likely to be the opening of divisions which the leadership would rather keep suppressed.

Mr Cameron's carefully worded statement describing his colleague's action as a "courageous but personal decision" is an attempt to seal off this gesture from the wider perception of the party - a daunting task.

He will also be privately furious that a senior colleague has disrupted a period of near-unassailability for the Conservatives by creating a distraction from the woes of Gordon Brown.

Mr Davis's replacement, Dominic Grieve is an impressive intellectual politician with a lawyer's grasp of detail.

However, he lacks the wily Mr Davis's skill for reflecting public anxieties about crime and his instinctive grasp of how to undermine Labour, product of many years' experience.

The peril of Mr Davis's coup de théâtre for his own party is that his one-man protest undermines the Tory unity Mr Cameron has striven so hard to build.

Mr Davis set out to highlight what he sees as a cardinal sin of New Labour. His own party may well bear the brunt of his crusade.