FOBS
We can rule out a terrorist organisation doing this, but the threat of a State actor deploying an EMP weapon is not an empty one. It might not be top of the list of threats, but Fractional Orbital Bombardment Systems (FOBS) have been known about for a long time. These have a strong deniability component and are also very difficult to counter. Civilian infrastructure would be very badly impacted. For example, food distribution is heavily reliant on just-in-time delivery and refrigeration, both of which would be badly hit by a significant EMP event. You don't need a multi-megaton weapon to cause a non-negligible EMP. In fact fission weapons give you more bang for your buck if you want to take out electronic infrastructure like telecomms and computer equipment. A big thermonuke will be better for knocking out power distribution systems. Both are hard to safeguard against.
It's easy just to dismiss this as lobbyists angling for cash, but it's a real problem.