Tim Kaine repeatedly defended Hillary Clinton’s use of a private email server during Tuesday night’s vice-presidential debate. “The FBI did an investigation,” he said at one point, “and they concluded that there was no reasonable prosecutor who would take it further.” But such a statement is credible only if it follows a real criminal investigation—that is, the sort of investigation that the FBI and the Justice Department conduct when they actually care about a case and want convictions.

We know all too well what that kind...