To some people, the administration’s posts sound patriotic. Others might sense at most a faint dog whistle to extremists. Some posts may just look odd. But those well-versed in the abstruse codes of right-wing extremism hear klaxons. The posts have referred to neo-Nazi literature, ethnic cleansing and QAnon conspiracies, mused about deporting nearly a third of the U.S. population, and promoted lyrics from an anthem bellowed by the far-right militants of the Proud Boys. Their authors are not on society’s fringe. They are in the offices of the White House and the departments of Homeland Security and Labor, using official...