Did a little surfing. Since the EMP effects are primarily line-of-sight, it's tough to see how anyone without a booster capable of hefting a 20 MT warhead to LEO is going to get the effect CONUS-wide.
I think the likely bad guys can, at most, knock out a metro region with what we're talking about.
I wasn't thinking continental US. Er, think globally, nuke locally? Nuke locally, act globally?
Something like that. :)