As Dudley explains, the evangelical position did not shift until much, much later. Dudley explains that abortion until the moment of quickening was legal in early America, and was not banned until the late 1800s when the medical establishment and prominent political leaders became concerned that the declining birth rate among Protestants meant the country was at risk of pending political takeover by prolific Catholic immigrants. Indeed, evangelical Christians were conspicuously absent from this campaign to criminalize abortion, preferring instead to devote themselves to the temperance movement with its desire to ban alcohol.
Beyond all this, Dudley reveals that in the late 1960s and early 1970s leading evangelical theologians were still arguing (in leading evangelical publications, no less) that, based on what the Bible says, the fetus was not a person with a soul. In fact, in 1968 an evangelical gathering hosted by leading evangelical publication Christianity Today called for the legality of abortion and in 1971 the Southern Baptist Convention formally called for abortion laws to be loosened so as to allow for abortion in the case of rape, incest, fetal deformity, and emotional, mental, or physical duress of the mother.
As Dudley carefully explicates, evangelicals argued that the fetus was not without value, and that its life should not be terminated without good reason, but they were unanimous in concluding that the fetus was not a person with a soul and that abortion was not murder. All of this changed in the late 1970s and early 1980s. By 1984, the previous accepted evangelical position that the fetus was not a person had become heretical and evangelicals began insisting that the Bible says that the soul is imparted at conception.
I like this passage quoted from the book:
In assessing evangelical thought on homosexuality, it is helpful to compare it to evangelicals’ thought on abortion, the other issue at the heart of their social agenda. The comparison reveals two different standards for deciding upon “the Christian position.” In the case of abortion, as we saw in the previous chapter, evangelicals have happily abandoned more traditional interpretations of the Bible and embraced creative reinterpretations of all kinds to claim the Bible teaches that life begins at the moment of conception. Even while evangelicals object to appeals made to broad themes in the bible to override the passages taken as condemning same-sex relations, they have appealed to broad themes in the Bible to override a strict construal of Exodus 21. Even while they have welcomed fallacious arguments from science to support new views on the beginning of life (claiming science proves life begins at conception, for example), they have objected to such arguments to support new views on homosexuality. Even while they object to emotional arguments from experience in the case of homosexuality, they have welcomed such arguments in the case of abortion.