Hillary Clinton was inartful in mentioning the assassination of Robert F. Kennedy on Friday, as she tried to explain why she is staying in the race for the Democratic nomination. But that's all she was: inartful. Clinton spoke with an eye on a dwindling political calendar and an awareness that the record is studded with lengthy Democratic nomination fights. Bill Clinton went all the way to June before winning the party's standard in 1992, and Kennedy was still in the thick of a race in June 1968. It has been a long time since the country has had a race...