If Hillary Clinton loses this election, it won’t be because of Bernie Sanders. It won’t be because of Marco Rubio or Jeb Bush or Carly Fiorina. It will be because of a 1966 statute.

The Clintons are street fighters, and over their scandal-plagued years they have mastered outwitting the press, Congress, the Justice Department, even special prosecutors. But the reason Mrs. Clinton isn’t winning her latest scandal is because she faces a new opponent—one she can’t beat: the Freedom of Information Act.

Of...