I find that hard to believe. Most of the Republicans I talk to think he was, but I'm not in the South
As the poll suggests, most Republicans outside of the South probably do think he was born in the US (maybe 75% or so). But Southern Republicans overwhelmingly believe he was not born in the US or are "unsure".
Yeah Republicans in the South are pretty notoriously stupid and populist on issues, especially when relating to race. I think this poll accurately shows how much of a grip the South has on the Republicans presently.