You might as well argue that since making the murder of the post-born illegal has not stopped the practice, it might as well be made legal. Before abortion was legalized, very few abortions actually took place. Read some of the things early legal abortion proponents have to say on the subject--wish I could remember the name, but I remember one guy saying that the numbers both on illegal abortions and women dying from the same were grossly exaggerated in order to sway public opinion.
As far as the "extremist" pro-life views driving away "moderate" Republicans, guess what triggered my move away from the Democrat party? If you guess "their extremist view on abortion," you've got it. Check the polls: abortion does NOT have majority support. Sure, the headlines on the polls will often claim that, but when you dig in and look at the numbers, you'll find that the poll's authors have lumped in people supporting abortion in the case of rape, fetal abnormality, or immediate danger to the mother's life (which together account for less than 10% of abortions) with those who support abortion as a form of birth control, presumably because the authors didn't like the actual results of the poll.
Furthermore, the only "religious" reason I can see for people to be against abortion is that murder IS one of the ten commandments, and thus is forbidden. It is science, not the bible, that tells us that the pre-born is a living human being. I have great faith in the accuracy of science.