On what evidence? Before the end of the resource wars, the European Commonwealth was a single superstate (Think of a natural progression from the current European Union), Europe has a very similar ideology whether you are from England or Greece. Trust me, NO European country would go East (at least not voluntarily). The breakdown of the European Commonwealth occurred after the oilfields in the middle east dried up at which point it became a 'survival of the fittest' mentality.
Now I understand that Europeans both want their post apocolyptic coolness, as well as some serious hope of something surviving there, but I just don't see it realistically (of course in fantasy anything can happen theoretically, but if your trying to be realistic... well things are grim).
But the US would survive? No sir, it would not. In fact, realistically Europe would probably have not been Nuked to the extent that the US was. Why? Because there are no resources left! What threat do they pose, they are too busy fighting each other! The UK is confirmed to be in a bad state but that's probably due to the resource wars rather than nuclear attack. Which is why Tenpenny is perfectly healthy and strong enough to survive a transatlantic journey. IMHO.
It's important to understand that the US has the abillity to intercept missles with submarines as well as the possibility of some kind of "Star Wars" system that may or may not secretly exist. The missles coming from quite a ways away. With Europe, you'd be dealing with guys firing missles right over a border. If say England and France went at it today, even if one side was winning conventionally, the loser would fire their bombs to take their enemy with them (Mutually Assured Destruction) and both sides would be like totally dead given the realitvely close proximity of each other.
The US does not currently have the ability to shoot down Nuclear Missiles, it is theoretically possible but you would have just as much chance by shooting flak in the sky. As for geographical distance, it is of no relevence at all, an ICMB traveling 100 miles or 5000 miles does not make a jot of difference.
We also have more nukes than anyone. At any rate, while DC would be a priority target for everyone going after the US, it would also probably be the most heavily defended in a crisis like this.
There are enough nukes in the UK alone to destroye the US. The worldwide number of nukes in 2008 is speculated to be enough to destroy the entire planet 6 times over. I can shoot you in the head once or 5 times but you will still be just as dead. Again, you can not effectively defend yourself from an all out nuclear attack.
It's all opinion at the moment but I think realistically Europe would be just as screwed at the States, but probably nowhere near as radioactive.