Well... that's the buzz, anyway.
Suppose Iran, or some non-state actor, got hold of a nuke and a way of lofting it? All it takes is one little nuke in the upper atmosphere to knock out everything electrical in the U.S., right?
Um. Let's take a look at an actual EMP event, shall we? Starfish Prime caused significant disruption (not total destruction) of electrical and electronic systems 900 miles away.
And that was a 1.4 MT bomb. Not a 14 KT fission bomb; a bloody great fusion bomb.
So... to create widespread destruction... our hypothetical enemy needs to use his hypothetical fission bomb that any serious industrial power ought to be able to build (but many seem to struggle with)... and use it to light off a megaton-range H-bomb, which is not widely regarded as being an easily-built item even given the fission trigger.
And then they need a missile capable of delivering that H-bomb (close to a ton, for a well-evolved design in 1958; likely much more for a first try) around 250 miles above the middle of the U.S. And, if it doesn't have the range to do that from their own territory, they'll need a place to launch it from - maybe a cargo ship; I don't think Cuba would want to play that game this year.
And, looking at the effects described for Starfish Prime, just one of those bombs would create a Big Annoying Inconvenience. What did the feller say about striking at a king? We'd have plenty of resources left to track down the perpetrator and retaliate memorably. A successful, properly crippling attack would need several H-bombs, and several missiles, and... well, basically, we're looking at Russia, or China, or just maybe France or England being able to pull it off.
And, somehow, I don't think anyone's been testing H-bombs without us and the Russians noticing - nor the big missiles, for that matter. So, the sneak attack by a new Power would involve a bunch of untested hardware. How good do we think their mathematicians and rocket plumbers are?