Instead of categorically defending a woman's right to an abortion, they have chosen to challenge pro-lifers on the pro-lifers' turf.
They have chosen to defend pro-lifers instead of defending a woman's right to abortion. 
mnli hypothesis: They have chosen to defend pro-lifers instead of defending a woman's right to abortion.  premise: Instead of categorically defending a woman's right to an abortion, they have chosen to challenge pro-lifers on the pro-lifers' turf.