Even if the DPRK fired a dozen nuke-tipped ICBMs at the US, they could not reasonably expect more than a handful to be effective and the response would be hundreds. Kim Jong Un could only be effective if he launched a dozen 2.3 Megaton EMP devices and one or two worked. They would not have to survive re-entry. Between one and three of those detonated 400 kilometers above North America would destroy the US infrastructure, and much of that of Canada and Mexico for a period of months or, more likely, years. After such an event the only country in the world with enough manufacturing capability to replace the US power grid within a couple of years is China, and only if they were in the mood to divert enormous resources to it. My guess is that they would immediately agree to do it, but that unavoidable delays would delay the process by additional years.
In fact, large ICBMs are not necessary or desirable for EMP attacks. A few solid fuel medium lift rockets fired from a container ship near the 200 mile limit would generate an EMP event within about ten minutes from launch, assuming a speed of 3 to 4.2 miles a second – not enough time to activate US civil defense measures, which would be ineffective in any case.
If any effective deception were in place, the US might have no immediate idea who to target for retaliation. A modified Russian submarine firing a modified Russian missile could have originated anywhere on earth.
DPRK flagged ships routinely make port calls in Cuba via the Panama canal, so DPRK ships could theoretically bring a load of missiles within 3,800 or so kilometers of Kansas, the ideal spot to detonate an EMP, and achieve a launch-to-detonation time of less than eleven minutes. The US knows it and so does the DPRK which has had an ongoing EMP research program operating for years.
In my opinion an EMP attack on at the US mainland by the DPRK (or any other potential adversary) from off the US coast, either as a first strike or as a retaliatory strike is a foregone conclusion and is a major reason the DPRK is pursuing a submarine launched medium missile.