If Vitter resigned now, Jindal would be able to appoint a replacement who would, effectively, be an incumbent for 2016. But Vitter and Jindal hate each other, so Vitter will not give Jindal the opportunity to (ed pick himself?) as a successor. The result is that the U.S. Senate 3rd class seat for Louisiana will be an open race in 2016. See full Analysis by Darryl Holman at HorsesAss.