I'm going to be in a minority here, but Stalin, having liquidated all coherent opposition, was, in many respects, the keystone figure of the USSR. The red court was every bit as bitter and divisive as its feudal predecessors, with a whole host of 'favorites' trying to get laws and positions past the General Secretary. There was no conventional, modern political process at work, especially among the upper echelons - it was pretty much survival of the fittest, and a strong leader was essentially neccessitated to keep everything intact (see the post-Lenin leadership struggle of the 1920s). Conflicts were left to simmer beneath the surface whilst Stalin held the reins of power. If he dies in the late-1930s / early-1940s, the daggers are going to be truly out of their sheaths.
There's an assumption on here that the Soviet bigwigs would somehow pull together in the face of German aggression at such a critical hour, because, well, they're Soviet bigwigs. This sort of situation has a track record, and it's not a good one. The USSR was not a state built upon reliable power politics. There isn't going to be some immediate leadership transition, nor can you trust distinct strands of the government apparatus, in the absence of a central force, to coexist meaningfully. Dodgy dealing and dirty tricks were modus operandi. Nobody's saying Stalin was a exceptionally able leader. He was, however, a unifying one.
BTW, this needs a move to After 1900.
