A new Daily Kos/Research 2000 poll finds that 77% of Americans believe President Obama was Indeed Born in the United States, with only 11% saying he was not -- but there's no clear verdict among Republicans.
Among Republicans, it's a much weaker plurality of only 42% who say Obama was born in the U.S., with 28% saying he was not, with a very high undecided number of 30%. Among Democrats, the number is 93%-4%, and among independents it's 83%-8%.
This means that for Republicans to openly admit that Obama is indeed a natural-born American citizen, they risk alienating a significant chunk of their loyal base. And on the other hand, they could scare away independents by humoring the tin-foil hat crowd.
Late Update:: Another thing to point out is that Birtherism is heavily concentrated in the South. Only 47% of Southerners say Obama was born in the United States, 23% say he was not, and 30% aren't sure.