The whole thing is based on the idea that fetuses and embryos are actually fully people like you and me, even though fetuses and embryos are not counted in the census, there's no funeral after a miscarriage, and people say "we have two children and one on the way" instead of saying "we have three children."
Now, why they consider fetuses and embryos to be fully people depends on the axe they have to grind. If you're a rank and file voter, you believe god can read every thought even before it's thought, so if at any time you even contemplate that fetuses and embryos might not be people, god will get you for that. God never forgets anything you ever did or thought, so you better stay on the right side of everything all the time and never slip even once, because otherwise Jesus will question whether you are truly saved and you may still end up going to hell hell hell. If you're a legislator in certain states or districts, you're basically trolling for votes from rank and file voter. There is some intersect between the two.