It’s is a largely localized issue. If you kick of an EMP over Kansas, it will probably only damage a few things directly under the blast. That’s what the Russians discovered when they actually did it.
I have read that if terrorists take out about 5 transformer stations, they could black out most of California.
Nukes by themselves make for poor emps. It’s when you undertake to create a huge inductance coil and a massive capacitor and saturate it with enough instantaneous energy to make the system go “snap” when you set off a high energy explosive full of charged particles that you might have something workable. The weight of such a system to create a continent wide crippler makes it impractical to launch. This is not to say emp’s couldn’t be employed in key areas to cause a large amount of damage and we should be hardening our systems. A potential solar event CME event for example, by itself should’ve been enough of a reason to have hardened our systems decades ago!