The Washington Post is reporting that Robert Mueller is now investigating President Trump for obstruction of justice, examining not only the president’s alleged statement to Comey in their February meeting, but also the firing of FBI Director James Comey. If true, this development makes the argument even more compelling that Mueller cannot serve as special counsel. He has an egregious conflict of interest. The special counsel statute specifically prohibits Mueller from serving if he has “a personal relationship with any person substantially involved in the investigation or prosecution.” The language is mandatory. He “shall” disqualify himself. Comey is substantially involved...