Unlikely US Event

This will fall into the "unlikely to impossible to happen" arena, but...
A thought recently raced through my head that I am sure the professionals have already prepared for. What would be the prime time to knock the US out of international leadership and possibly knock the US out period?
A group, or individual, fanatically opposed to the US with financial backing of tremendous wealth acquires a loose tactical or back pack nuclear device (reference previous entry Border Worry Dec. 21, 2007)left over from the USSR or elsewhere. This person or group places the device near the Capitol Building in Washington, DC and detonates it on, say, Jan 20, 2009. What effect would that have?
The US would lose:
1. The out going president and vice president, their staffs and families.
2. The in coming president and vice president, their staffs and families.
3. The entire US Congress and their families and staffs.
4. The US Supreme Court and their families and staffs.
5. The Chairman of the Joint Chiefs of Staff and the Chiefs of Staff of the US Military.
6. Visiting foreign dignitaries and their families and staffs.
7. Numerous state governors present for the inauguration.
8. Elite members of our military, law enforcement, and emergency responder agencies.
9. Millions of Americans present for the event.
What lunatic jihadist with access to middle east oil dollars would not jump at the opportunity?
That would be a completely devastating blow to the country without doing damage to the national infrastructure. The radiation would be blown out to sea, and the gulf stream would carry it out, and away from the US.
There would be no national government left. Martial law would be ineffective with so many soldiers overseas and with the unified command gone. Near main infantry bases there could be some degree of control, but that would not extend far from the base boundaries. It would be the effective end of US foreign policy and we would have forces stranded overseas. Criminals would seize the opportunity to wreak even more havoc and anarchy would prevail where there was not a strong local government to quell it.
Unlikely, yes. It was unlikely that the Spanish would sink the USS Maine in Havana Harbor. It was unlikely that one torpedo would sink the Lusitania. It was unlikely that the Japanese would attack Pearl Harbor. It was unlikely that some undereducated Muslim radicals would fly planes into the Twin Towers.