If Labour were to lose London and suffer heavy loses elsewhere, then the steady flow of stories about how the Brown government is in terminal decline will become a torrent. It is increasingly hard to see how Brown could turn things around in these circumstances.
The other problem for Brown if Labour does badly on May 1st is that he would be under fire from both the left and the right. The reformers would argue that he had allowed Cameron to capture the common ground while the left would say that he needs to let Labour be Labour and draw clear dividing lines between himself and Cameron.
It is rather ironic that Brown finds his fate so much in the hands of Ken Livingstone, a man that Brown can not stand. But if Ken is Leavingsoon then Brown probably will be in 2010.