Source? The highest percentage I've seen in any poll was around 30%.
I know several Obama-hating conservatives who mock "birthers." And I seriously doubt that anyone who voted for him in 2008, a majority of voters, even if they hate him now, believes he is not a US citizen.
I know lots of people that voted for the puppet in ‘08, knowing full well that he is not a citizen, and celebrated that fact.
Obama voters are just as mendacious as he and his controllers are.