Slop.

From what I've read back in my atheist activist days it coincided with integrated schools as a culture war issue becoming untenable as a position so they moved on.

There were always Christian groups against it, Catholics were required to be against it because of a non-revocable Papal decree in the form of "received wisdom from god" type was announced at some point in the 20th century I think. Some Protestant sects were against it, others shrugged at it. There were different theories for centuries on when the fetus was given a soul so different time periods where it was okay. Some traditional religions for example thought the soul was inhaled on the first breath after birth, others gave 30 or 90 days and for much of history abortion was considered okay as long as no movement had yet been felt.

So basically they grabbed onto this as a culture war issue. You know that Republican strategist who wrote in his memoir about how you used to say n-word n-word, etc and then you couldn't do that you moved onto integrated schools then busing and soon it was all an abstract thing but racism just the same. It's the same thing but for sexism, they couldn't be against sexual freedom that second wave feminism had brought and won and which was quite popular, at least they couldn't do so while winning without some wedge issue (they still were openly against feminism but a lot of type folks shrugged at all that and didn't care to join their movement). So they couldn't just slut-shame and call for criminalizing sex outside of marriage, it wouldn't fly and wasn't going to succeed. So what do you do instead? You punish women with "consequences" in the form of having kids. It becomes abstract, you invent dishonest tactics and claim they're killing babies and do this whole moral masturbation thing and freak-out over that, that plus trying to keep contraceptives to couples and you clamp down on sexual liberation by making women liable to suffer pregnancies and bear children they may not want as punishment for sex and then go on and on about stable families and births in wedlock and you're doing the same slut-shaming thing but it's abstract, you're talking in sociological terms and of murder and stable society and families and its cryptic.

And what happened was the school integration and open racism card was no longer as useful by a certain point so political operatives latched onto it after prominent "moral majority" Evangelical leadership did (who themselves of course were deeply political creatures) and they turned it into a culture war issue, they made it part of their purity culture push but they also pushed it beyond that. After all if you can convince people that abortion is murdering a baby you don't have to sell them on your whole system of morality, they don't have to accept the purity culture as a whole because they can delude themselves into thinking oh the women will give it up for adoption, there are plenty of families who want it, it'll be fine for the child and there wasn't much further thinking on it or these poor misled fools would sit there and cry and be upset about the situation because blah blah murder is wrong, poor kids, boo hoo. And they won over a lot of women that way and continue to unfortunately.