David Davis's shock decision to resign from the Commons and fight a by-election over 42-day detention is, ostensibly at least, designed to mount a challenge to the moral authority of the Brown government.
In the longer-term, it could achieve just that. If Mr Davis is successful, it will explode the Prime Minister's claim that there is public support for the measure and make it much harder for Labour to use the Parliament Act to force the measure through the Lords.
But without doubt, this decision also has to be seen as a severe blow to David Cameron. It is clear there has been some almighty bust-up between the Tories' two main men, and as a result Mr Cameron's authority will now be seriously called into question.
Davis was also the best-performing member of the Shadow Cabinet by a mile and has consistently made all his opposite numbers at the Home Office appear "unfit for purpose" in John Reid's immortal words. If this is the end of his frontbench career, it will be a sad loss to the party - and potentially to the country to.