According to the US EMP Commission (2004):

"Several potential adversaries have or can acquire the capability to attack the United States with a high-altitude nuclear weapon-generated electromagnetic pulse (EMP). [...] EMP is one of a small number of threats that can hold our society at risk of catastrophic consequences. EMP will cover the wide geographic region within line of sight to the nuclear weapon. It has the capability to produce significant damage to critical infrastructures and thus to the very fabric of US society, as well as to the ability of the United States and Western nations to project influence and military power."

Other questions in this tournament asks how likely a HEMP attack is by 2024 and how many such attacks would occur if at least one does (link will be added soon). This question is about how much harm would occur if one or more HEMP attacks occur. For further context on this question, see Nuclear risk research idea: Nuclear EMPs and [rough notes] Harms from nuclear conflict via EMPs, fallout, or ozone depletion.