A reader writes:
Changes in the direction of greater support for the "pro-life" position is likely due to the success of legal and safe abortions. Abortion has been safe, legal and rare for nearly 40 years now, and so the majority of folks have no context to understand what a "pro life" world really was like. They have the luxury to contemplate the question as some sort of hypothetical. Should abortion become illegal and/or difficult to access, you'll see a upward spike in the "pro choice" numbers detectable within months, not years.
To me, there is a parallel here to the anti-vaccination crowd.
You have to be over 40 (at least) to have any understanding of what life was like before vaccines. Maybe you didn't know anyone who died of measles, but you may have had a friend whose father was a polio victim. Anti-vaccination advocates have the luxury of never seeing the implications of their own position and so, beyond the the debates you may see online or on day-time TV, their position gets little consideration. Similarly, "pro lifers", for the past 40 years, have not had to witness the consequences of their position. Additionally, younger folks (<40 years old) can't possible have a complete context for the abortion debate because they didn't live in the illegal, unsafe and rare world that preceded 1973. Should we ever end compulsory vaccinations or make abortion illegal, you'll quickly see polls that basically say, "What the hell were we thinking?!"