One danger for the anti-Brown forces is that their action is presented as a Blairite putsch, with James Purnell following Hazel Blears from the Cabinet. But that may be a more difficult line to run if Peter Mandelson is to remain perhaps the most valuable remaining protectors of the Prime Minister.
The most interesting part of Purnell's resignation letter is the deliberate rejection of those factional dividing lines.
This moment calls for stronger regulation, an active state, better public services, an open democracy.
It calls for a government that measures itself by how it treats the poorest in society. Those are our values, not David Cameron's.
One thing that has not often been understood about Purnell has been that he combines advocacy of a modernising and unabashedly New Labour agenda with a 'next generation' rejection of the media's definition of what Blairism or "uber-Blairism" are about - his best line has been that people forget that New Labour was Labour as well as New.
Despite Purnell's Blairite credentials, which have seen him often championed by The Spectator, he could legitimately claim to have put the Labour argument on poverty and inequality as clearly as any member of the Brown Cabinet. (Most often on Fabian platforms, as with his lecture on child poverty after last year's local election results, proposing that this could be a fightback theme to could unite Old and new Labour, which has also a hope of many supporters of Gordon Brown.
Indeed, last September Fabian Review awarded Purnell, slightly tongue-in-cheek, Keir Hardie's flat cap because of the strength of his comments on redistribution and claiming an Old Labour pedigree for conditionality.
The other intriguing rumour in recent months has been of a Purnell-Cruddas future alliance along these lines - though Jon Cruddas has made it categorically clear that he is not involved in any moves to destabilise the Prime Minister.
It has long been thought in Westminster that James Purnell and David Miliband have an understanding that they would not run against each other in a future leadership contest. Purnell's resignation letter seems to be ruling himself out of a contest, and denying any coordination. (The idea that there is a coordinated plot is undermined by the loyalist interviews which have been given by 'Blairite' ministers such as Caroline Flint, Liam Byrne and Jim Knight in the last few minutes).