How about...
2001: George Bush becomes the latest president to die in office, however his death is not from the assassin's bullet, but a rogue pretzel lodged in his throat. Dick Cheney is sworn in as POTUS shortly after
2001-2004: Cheney's presidency quickly loses it's initial popularity especially after Iraq is invaded and quickly bogs down into a "quagmire". Cheney's poor health coupled with the unwillingness of his VP Condoleesa Rice to run in 2004 leads to a wide open race in 2004. Not facing an incumbent popular president, Hillary Clinton launches a successfull campaign to take the White House.