Testifying last week before the Senate Judiciary Committee, James Comey recalled a moment that should have held more significance for him than it did. At the height of the presidential campaign, President Obama’s attorney general, Loretta Lynch, had chosen to meet with Bill Clinton on an airport tarmac. That, said the now-former FBI director, “was the capper for me.” Hillary Clinton’s emails were being probed, but Ms. Lynch was too conflicted to “credibly complete the investigation.” So Mr. Comey stepped in.

Donald Trump...