Yes, the EMP fearmongers are at it again, boldly asserting that a small nuke set off above the middle of the U.S. could destroy everything electrical... with new added claim that the ensuing collapse would kill 90% of the population.
Yeah, knocking out civilization is serious stuff; couple decades back, I took a look at the implications of evacuating the cities ahead of a nuclear war, and quickly concluded that fallout would be a much smaller threat than lack of sanitation for a large displaced population.
Let's take a look at this "one little nuke" thing, shall we? Starfish Prime was 1.4 megatons, not exactly little. It was significantly disruptive 900 miles away. The one bomb in the middle needs to do a lot of damage at the coasts; call it a 1500 mile radius. So, assuming we're dealing with an inverse square law here (and not an inverse cube or something), we need to multiply by (1500/900)2, so about 4 megatons. But! That's to inflict the level of damage at the coasts that Starfish Prime inflicted on Hawaii, not to wipe out the electrical grid, backup systems, car electrical systems, and all that other stuff. So, we probably need another order of magnitude, to get enough energy for a proper kill.
Which says out hypothetical enemy needs Царь-бомба, or something darn near. Plus a delivery system to deliver 20 tons or so of bomb to a point a couple of hundred miles above Omaha. Which is a damn big ICBM. Smaller than a Saturn V, but... well... somehow I don't think the NorKs* will have one anytime soon. And we'd sure notice them testing it. Not to mention the seismic activity from the giant H-bomb tests.
And, yeah, the bomb and its delivery system would need testing. Because it's not just rocket science; it's rocket plumbing. And when you strike at a king....
None of which is to say that we shouldn't be hardening the electrical grid, having decentralized emergency supplies, and otherwise preparing for disruptive events whether hostile or natural. But I call BS on this particular bit of fearmongering.
* The Russians may have both the bomb and the delivery system (it's just at the upper limit of Протон-М's capacity). But Tsar Vladimir is ambitious, not insane. Launch an ICBM from Kazakhstan to deliver a huge nuke over Nebraska, and, regardless of who's in the Oval Office at the moment, you just don't bet against several Russian cities becoming fields of rubble within the hour.