I can't imagine for a second how they think standing David Miliband would in any way be a Good Thing.
Are they really that utterly oblivious to their membership and the unions? Not only is he a hark back to the good old Blairite days of yore, which frankly should be disavowed and disowned once and for all, but parachuting someone in for the leadership race would surely turn even Corbyn's haters against them. The membership do not like parachutes.
I just can't see how they'd work that one out in their head as in any way a good idea.