 This is CBS News on the Hour, sponsored by O'Reilly Auto Parts. I'm Vicki Barker. No matter who wins or loses, tonight's presidential debate will go down in history as the first between a current and former U.S. president. We have team coverage of tonight's showdown, beginning with CBS's Linda Kenyon. This debate is a first for several reasons. We've got this unprecedented situation of a former president running against a current president, and this election is sort of