A widespread EMP attack would require nuclear weapons and would constitute a strategic nuclear attack against the US directly threatening vital US interests. US defense policy is pretty clear. The betting man would go all in on the per being a goner.
Let’s say the EMP warheads were launched from Submarines off the east and west coasts in international waters. No country has any defense for that, going further, what if the EMP effects were only along the east and west coasts maybe 100 miles inland, it would be devastating and yes, we could respond with a nuclear response, then what at that point it’s Armageddon.
Russia and China likely not going to risk the response, what about Iran or North Korea, to say they don’t have the ability is naive IMO, they can get the ability from Russia or China.