Birthers make a majority among those voters who say they’re likely to participate in a Republican primary next year. 51% say they don’t think Barack Obama was born in the United States to just 28% who firmly believe that he was and 21% who are unsure. The GOP birther majority is a new development. The last time PPP tested this question nationally, in August of 2009, only 44% of Republicans said they thought Obama was born outside the country while 36% said that he definitely was born in the United States. If anything birtherism is on the rise.