Can you use a single nuke to EMP the entire continental US?
No, not really. If you had an EMP that could cook via electromagnetic radiation the electrical grid of the US, you'd have worst problems than EMP caused black outs to deal with.
The whole idea of an effective EMP is to fry as much cooper/aluminum wire as one could. Think of really effective EMPs being more like lighting and less like nuclear detonations, since using a nuclear detonation is like trying to cut off the kitchen lights using a bulldozer and thirty tons of sand. If we're strictly talking EMP, lighting and all the static discharge family is your better bet.
So with that said, a lot of electrical companies are prepped for pretty bad EMPs with response teams, though I'm pretty sure most of America would find that hard to believe when their power does actually go out. One thing that is actually going to be slightly more difficult to deal with is first finding the point of failure, bring it down, and then put up the replacement. With a bomb, it's pretty easy to figure out where the failure is and the upshot is that the part to be replaced is already on the ground or missing completely so you can skip that whole removal step. Yeah, we might be talking several kilometers needing to be replaced with a nuclear device, but you know from the first second you need a couple of hundred kilometers of wire. A good EMP keeps you guessing and has a dozen or so employees walking hundreds of kilometers of wire trying to find the failure. Better sections of the US grid have more fine grain reporting points so maybe on a dozen or so kilometers need to be checked. However, the actual transmission lines are the key to a good EMP. That said, you don't need a big "bomb" to be effective, you just need coordinated attacks on major transmission lines. However, doing that alone is just more of a major disruption, rather than a major blow to the nation.
Additionally, power generating plants usually have a lot of counter measures for EMPs. So you really aren't going to take out the generators. It's silly to think that someone could without a massively coordinated attack. Especially if we're talking strictly EMP here. If we're talking a nuclear device, again, you've got bigger problems especially if you had enough to take out all the major plants in the US.
The real danger here, I guess, is consumers. Some EMPs can fry pacemakers and pretty much anyone on life support is dead in a massive EMP. However, it's not the end of society and more so, hardly the end of the US. You can take any example of when some large section of the US had power knocked out for several weeks. A massive EMP would be roughly equal to a hurricane without any advance notice that hit a large section of the US. If you were lucky enough to hit the entire US then multiply the figures in your head by that amount. However, the end of days for the US, hardily. EMP weapons in real life would cause some death but for the most part the US would be tired of martial law long before the US fell under the pressure of an EMP weapon.
The real tactical value of EMPs is not as some silver bullet, but as a disruptive force that is soon followed by other forms of attack. Additionally, you'd want your EMP to be as quiet as possible and look as much like a lighting strike as possible on the grid. Anything else and there would be way too much attention drawn that would get people ready for the obvious next strike. Thinking that an EMP would be a good primary strike is silly. Additionally, some have thought about EMPs in asymmetrical warfare contexts and while they would play a good role in the demoralizing aspect of that, it's just simpler to buy a ton of fertilizer and diesel fuel as opposed to trying to construct something massive enough to disrupt more than just a few dozen people. In other words, the reason low tech seems to win in asymmetrical warfare and terrorist operations is that you get more bang for buck so to say.
I think when you consider it carefully, those who would toss EMPs around like we are under some imminent threat of them, are doing nothing more but trying to push some agenda. EMPs just really aren't that great of a weapon and the technical curve to building ones that would be worth the time and money is just too high for your casual mayhem makers.