The Aug. 8 search of former President Donald Trump’s home and the unprecedented criminal investigation into his possession of White House documents all began with David Ferriero, the now-retired national archivist who alerted the Justice Department after finding classified information in boxes he retrieved from Mar-a-Lago. Critics say Mr. Ferriero is a partisan who changed the rules to help excuse Secretary of State Hillary Clinton when she was under scrutiny for mishandling classified documents on her private, unsecured email server. They also say the raid for a criminal investigation is just another politically motivated scheme to take down Mr. Trump,...