No, to attack the US, they will use something far easier: an ordinary seagoing cargo container (or multiple containers). I'm no electronics whiz, but I bet I could put together the package to set off the container-bomb during it's travels.
If you consider thousands of containers per week, arriving at dozens of US ports, and very little in the way of inspection, you begin to see the enormity of our vulnerability.
Very shortly in this Nation, we are going to be in a Depression (at the least). We will hardly be able to afford to keep soldiers in foreign lands while we are so strapped for cash. My suggestion will be to militarize ALL the ports, and inspect EVERY cargo container which comes through those ports. That will employ a LOT of troops, and the command structure of the military makes little difficulties with unions, etc, a moot point.
We CAN protect ourselves here.
Do we have the will to protect ourselves?