Damiangate had, through the course of today, taken on its own, awesome momentum. The options for No10 were clear: if McBride stayed, then tomorrow's newspapers (and Monday's, and Tuesday's) would be choc full of grim stories about black arts, smear tactics, hit men - a real diet of super-ugly news. And it wouldn't be Tories on the attack, but the likes of Charles Clarke and the many other Labour figures who have a score to settle with McBride. There's no sight more vicious than a Labour civil war.
So McBride has done a Bob Quick (but without the £110k pension). His motivation is clear: go now, and he will minimise the bad coverage - and, most importantly, there's a chance of rehabilitation by the general election campaign. Meanwhile I suspect he'll pop up in a Labour-related institution: a union (like Whelan) a think-tank (like Twigg) or somesuch. So this is not goodbye to McBride, but au revoir. He's too effective for Brown to axe completely.
* Ask Charlie Whelan, who hit the rocks during Christmas 1998.