EMP may fry SOME unshielded electronics and some electrical systems from power surges.
To cover the whole US, it would need to be a multi-megaton weapon detonated a hundred miles up, or a bunch of hundred-kiloton weapons spread over the country. It would produce damage to electronics that are turned on, less damage to stuff that isn't on. Car computers are mostly shielded by the metal bodies of the cars.
We may lose a lot. But anybody who detonates a nuke above the US will lose everything.
Uh....not necessarily. It depends upon which country attacked us. If it was a Muslim country, our current Muslim CIC would probably just let it slide.