It has been made plain to us that a American city can be totally devastated by a natural disaster. There is also the threat that a city might be leveled by less than natural means. On 9/11, an attempted decaptitation strike took place against the United States. The pilot who crashed into the Pentagon chose it as a secondary target, his primamry target would have been the Capitol or the White House. Had all four plane hit their targets, and had the president been at home at the time, this nation would have been in chaos. The last time a president was assassinated, nessesary functions ground to a halt on all levels of government.
The best way to prepare for this contigency is for states to be better able to funtion independently in the case of a temporary abscene of federal leadership. Not only will we be better prepared, it will make assassination attempts less likely. Terrorist and enemy nations would be not be as tempted to try to take out the President and Congress if they knew that the US would make a quick recovery.
It would be like shooting a rhino with a BB gun. You would only piss it off.