I believe that the route to "winning" the abortion argument is as follows:
First, acknowledge that all abortion, regardless of trimester or means of conception, is homicide, literally, the killing of a human being.
Second, recognize that most pro-lifers, as well as pro-choicers, understand and accept that some homicides are justifiable; examples include war, capital punishment and self-defense.
Third, establish a simple algorithm for determining whether an abortion is justifiable or illegal homicide based on which trimester. Leave that decision exclusively to the pregnant woman in the first trimester, but create strong rules for the determination in the second two based on the medical threat to the pregnant woman's life.