As J.S. Mill pointed out in his writing on liberty, people usually do not follow consistent principles. Instead, they act based on likes and dislikes, which often arise from misinformation and disinformation. Comparing the view of many Republicans on abortion to their view of on immigration illustrates this clearly.
To use a concrete example, Alabama passed a very restrictive anti-abortion law that even forbids abortion in the case of rape. Proponents of the law, such as Alabama governor Kay Ivey, claimed the motivation behind the law is to protect life. As the governor said, “to the bill’s many supporters, this legislation stands as a powerful testament to Alabamians’ deeply held belief that every life is precious and that every life is a sacred gift from God.”
On the face of it, the principle is that because each life is precious and a sacred gift, if a man impregnates a woman (or girl), then she is obligated to host the zygote until birth. The expenses and risks of doing so fall on the woman (or girl). The United States government does little to assist pregnant women. Looked at in the abstract, the principle is that if a child manages to get inside a certain area, then there is an obligation on the part of the owner of that area to care for that child until the child can safely exit the area. If removing the child would kill or harm the child, then the child cannot be removed, perhaps regardless of how the child got there.
This principle would seem to also apply to migrant children who enter the United States, even if they are brought here illegally and against the will of the United States. Once they get within the United States, if expelling them would lead to harm, then the United States is obligated to care for them until they can safely exit the United States. After all, if the principle permits compelling women to bear a child from rape, then it surely permits compelling the United States to care for migrant children who are here illegally. At least until the children can safely leave the country.
It could be objected that abortion always kills a child while expelling a migrant child from the United States will probably not kill them. Hence the analogy fails. One reply is to argue that if every life is precious and a sacred gift, then even harming a precious, sacred gift would be wrong. That is, the principle isn’t “killing them would be wrong, but anything else is okay” but that each precious life must be treated as a sacred gift and one does not throw a sacred gift out.
But making the strongest analogy requires considering only cases in which expulsion would result in death. There are migrant children (and adults) who are likely to be killed if they are sent back. It could be countered that, unlike abortion, they do have a chance of surviving. If so, the principle would have to be “each life is precious and a sacred gift, but this only entails that children should not be exposed to certain death. Likely death or great harm is morally okay.” While this is certainly a principle that one could hold, it is not commendable.
As such, there would seem to be two options for anti-abortion folks who also want to be anti-migrant. The first is to consistently apply their avowed principle and accept immigrants when their expulsion would be likely to result in their harm. Or, if they want to be extremely strict, their deaths. The second option would be to abandon or modify their principle so that it applies only to abortion but not to migrants. The challenge is doing so in a manner that is not ad hoc or begs the question. For example, just saying that the principle only applies to the bodies of women but not to the United States would be ad hoc, as would saying that only zygotes deserve to be protected. It is worth noting that those who are pro-choice and pro-migrant would also need to consider the possible conflict between their principles as well.
One might also suspect that some anti-abortion folks are not motivated by a desire to protect “sacred, precious life” but have other, less laudable, motivations. Such people could easily reconcile being anti-abortion with being anti-migrant, perhaps by holding the principle that the state’s power should be used to harm people they dislike.