I've read that the GOP's biggest fear actually is Roe v. Wade being overturned. For now, with no fear of any real action being taken to make abortions illegal, pro-choice voters (still a majority) will vote for Republicans.

Anyways, though, the real problem is social pressure. In some circles there is pressure not to have an abortion. The sad thing about Bristol Palin is that it's possible that she's being used as a political pawn. "Look, my 17-year-old daughter got knocked up and I'm not allowing her to get an abortion!"