In 2004, Kerry won voters aged 65 and older 54%-46%. In 2008, McCain won this group (which, lets keep in mind, is not really the "same group" four years later) 65%-32%. That is a massive swing, and I would say it accounts for a lot of the overall swing. It was probably a combination of Yellow Dog Democrats dying or staying home and a very, very significant portion of ancestral Democrats rejecting Obama (McCain won almost 30% of White Democrats in Arkansas).
Clinton's '08 polling in the context of Bush's low approval ratings suggested that many Bush voters in the country were ready to back the classical Clinton coalition, but Obama implemented an alternative strategy focused on high turnout and realigned the electoral map to a reinforced Gore/Kerry map.
