Considering the vitriol I have witnessed, not only are candidates fair game, anyone publicly supporting a candidate is fair game as well. In that sense “digging up dirt” does not seem to be an odd phrase IMHO.
The sad thing is it will get worse.
From that perspective perhaps not, yet Mark is a lawyer and a careful and wordsmith, his books are a joy to read, so it surprised me.
The whole incident among others proves that this election is a knife fight to the finish.