Special counsel Robert Mueller’s investigation may face a serious legal obstacle: It is tainted by antecedent political bias. The June 14 report from Michael Horowitz, the Justice Department’s inspector general, unearthed a pattern of anti-Trump bias by high-ranking officials at the Federal Bureau of Investigation. Some of their communications, the report says, were “not only indicative of a biased state of mind but imply a willingness to take action to impact a presidential candidate’s electoral prospects.” Although Mr. Horowitz could not definitively ascertain whether this bias “directly affected” specific FBI actions...