I'm more worried about the human element than the actual WMD attack. A serious attack would trigger a panic that would be just as bad, or probably a lot worse, than the attack itself.
Remember the incident with the envelopes with white powder in the mail back in late 2001? That had the entire country was on edge, far out of proportion to the actual threat. Only a few people were hurt, but everyone was freaked out. My neighbor's utilities were disconnected for non-payment. He was too afraid to open his bills and pay them.
Remember the DC beltway sniper mess? Half a dozen people got shot, but the entire area was a huge mess. Businesses closed, people were afraid to drive to work for fear of needing to buy gas.
A serious WMD attack could paralyze the country for a few days or even a few weeks. That, in turn, would destroy the country. The economy would stop completely. No trucks delivering food to cities. No trains delivering coal to power plants. Water supplies could be contaminated. Entire communities of people could become desperate, and therefore violent. Your neighbors would convince themselves that it's not fair that you are halfway prepared for an emergency and they aren't, and that it is right for them to take what's yours, because they "need" it.
The military would be the only organization with any sort of working infrastructure. They could provide assistance, but there just isn't enough military to go around in a country as big as ours. Hopefully the rest of the world would still have an economy, and hopefully they'd be willing to return some of the generosity that the US has shown them over the centuries.
