Congress could easily protect much of the grid by requiring power transmission lines that cross state boundaries pass through an AC-DC-AC conversion with trip/trigger active on those lines (basically and industrial scale surge protection). This would stop the power surge from an EMP blast at the state boundaries limiting the damage to roughly a 4 state region for one, even massive, EMP. I should point out that the solution would not be cheap.
However, it is possible for multiple EMPs in the right locations to take out the entire grid even with the above design.
Much cheaper to shoot down the missiles though.
Much easier to off the nut case threatening several countries.
There exist other reasons to implement something along these lines. EMP from high altitude nuclear blast may or may not be as dire as some claim, but a massive coronal expulsion resulting in sufficiently strong X class solar flares hitting Earth certainly would for the entire portion of the planet facing the sun at the time, I believe it’s approximately 1/3. It’s happened before within US history. It’ll happen again.