McCain 2008-Obama 2012 voters

[The Right Honourable Martin Brian Mulroney PC CC GOQ]

How common were they? What were their demographic tendencies? What tended to motivate their votes?

Some Hispanic voters. Specifically, Miami-Dade swung Obama's way in 2012, with Romney getting about 30k votes less than McCain, and Obama improving his margin by about 40k - so it seems like a lot of McCain voters went for Obama. This lines up with the fact that Hispanic voters as a whole swung blue in 2012 unlike almost all groups. Based on this, my guess would be Cuban-Americans who had backed Bush twice and distrusted Obama because of socialism accusations, but approved of some of his policies like the ACA and didn't like Romney

[The Right Honourable Martin Brian Mulroney PC CC GOQ]

Two notable McCain-Obama contingents were Alaskans (the state with the biggest Dem swing between the two elections) no longer distracted by having one of their own on the ticket

Alaskans didn't necessarily go from McCain to Obama in any large number. Obama's raw vote total in 2012 was roughly the same as in 2008, but Romney's total was nearly 30k less than McCain's. Alaska swung left, not because Obama picked up a lot of McCain voters, but because Romney didn't.