There is a reason Republican presidential candidates have an incentive to support birtherism. Their party’s base believes Obama isn’t a native born citizen at a much higher rate than normal Americans. For instance, check out this CNN poll. 72% of all respondents believe that Obama is “definitely” or “probably” born in the United States (a number that is too low, but I digress).
On the flip side, that number craters down to 52% when you ask just Republicans. By comparison, 89% of Democrats and 74% of Independents accept the factual information. The numbers are similar for tea party supporters.
When Republican members of congress and presidential candidates pander to birtherism, they’re actually appealing to their base.