Republicans have to be extra careful. If they give credence to the birthers, they're (not only advancing ignorance but also) betraying the narrowness of their base. If they dismiss this growing movement, they might drive birthers to find more extreme candidates, which will fragment a Republican political coalition.
So far, it seems that some Republican officials are quite happy to court the nutbase movement. Watch Chris Matthews evicerate Congressman John Campbell (R-CA), who eventually does admit that Obama is a U.S. citizen: