Last year, before Hillary Clinton's secret email system became publicly known, Congress passed a law to keep presidents from trying the same trick. If Clinton wins the White House, the law could well be put to the test. The statute is the Presidential and Federal Records Act Amendments of 2014. It recognizes that government officials sometimes (or in Clinton's case, all the time) want to use private email accounts — in the words of the law, "non-official electronic messaging accounts" — to conduct government business. Such communications are still federal records, Congress declared, and must be preserved in accordance with...