3/4 of London Labour MPs not backing Livingstone

I have only just noticed this letter in today's Guardian. Orchestrated, round-robin letters of support for a politician are, in my experience, normally a sign of someone in desperate trouble, especially when the letter is to go in a sympathetic publication like the Guardian is for Labour. I remember in Autumn 2003 being asked to co-sign a letter from parliamentary candidates to go to the Telegraph in support of IDS. Today's Guardian letter - signed by 10 London Labour MPs, including at least a couple of PPSs and a Whip - begs this question: why won't the other 34 sign?