Why restrict your analysis to such a narrow scope? Abortion was illegal in every state in the US by 1910. It’s also still illegal in my home country, which is a Muslim country on the other side of the planet from the US. Opposition to abortion is longstanding across completely different cultures and religions. It’s not credible to say it’s something that arises from evangelical Christianity or a late 20th century political movement.

What you’re describing is instead an ideological schism within Protestant Christianity. Since Luther, Protestant Christianity had a strong natalist orientation. That’s why abortion was illegal in every US state by 1910. But during the 20th century, mainline protestantism adopted an anti-natalist view out of concerns about overpopulation and the welfare of the poor. That’s the genesis of the modern US abortion debate.

It’s not accurate to say that these patterns reflect views on abortion being “fluid.” Protestantism isn’t centralized—especially denominations like Baptists. Until the mid-20th century, pretty much all the denominations agreed abortion was wrong. Once the anti-natal movement got going, those denominations fractured and people sorted into different ones. For example, the Evangelical Presbyterian church split from what’s now the PCUSA in 1981. Both are “evangelical.” But we apply the “evangelical” label to one and not the other, based on their views on abortion and other issues.