Consider the following argument.
1. A fetus is a person. (A claim anti-abortionists like to make.)
2. Killing a person is murder. (Leaving out some specifics, everyone grants this.)
3. Therefore, abortion is murder. (This is a conclusion following from 1 & 2 that anti-abortionists also like to draw.)
But let's continue the argument.
4. One definition of "first-degree murder" is the "killing of a person that is deliberate and premeditated." (The legal definition will vary from state to state, but this is pretty basic.)
5. Abortion is a premeditated act. (This is obvious. Accidental or spontaneous abortions occur naturally, but here we are talking specifically about the medical procedure.)
6. Therefore, abortion is an instance of first-degree murder. (This follows from 3, 4, and 5.)
You probably see where this is going now.
7. Those who commit, are tried, and convicted of first-degree murder typically face penalties as extreme as life in prison or execution. (This again varies from state to state.)
8. Therefore, as an instance of first-degree murder, women who have abortions should be subject to penalties up to and including life-imprisonment and death. (This follows from 6 & 7, although I'm being a little loose with my language here.)
Now, 8 is a consequence that few pro-lifers are willing to admit. Let's assume that they're at least minimally consistent and also oppose the death penalty. They would still be reluctant to want to put women who have illegal abortions (for instance, in South Dakota or Mississippi now) in prison, and certainly not for life.
Doctors who perform these procedures (in essence, the equivalent of hitmen) are subject to up to 5 years in prison under the South Dakota statute, I believe--but this is still well short of what a first-degree murder charge would confer.
Why do I raise this argument all of a sudden? Because of this video (h/t Digby).
While perhaps half the country calls themselves pro-life, a fair number still don't favor outlawing abortion. The people in this video all do.
But what's amazing is that despite dedicating years of their lives to this project, they've never thought once about the practical implications. One elderly woman in the video argues in a circle and clearly contradicts herself. It's funny and sad.
This is why premise 1 is ultimately implausible. Further reinforcement is found in this example (h/t Leiter), in which a right-wing radio host is asked whether he'd sooner save 5 petri dishes or a toddler in a burning fertility clinic. And while you may call something a "person", if you don't treat it like one, you are just being dishonest.
For a while, I had been working on trying to develop an argument in favor of the legalization of abortion that acknowledged, for the sake of argument, that a conceptus is ensouled (which seems to be the only plausible basis for confering personhood from the moment of conception).
Part of my argument was based on the finding that over half of pregnancies end in spontaneous abortions anyway. What happens to all those souls you might ask? (It depends on who you ask. The Catholic Church recently abolished Limbo so who knows where they all went. If you're a really nasty fundamentalist and think that only people who accept Jesus get to heaven, then they're all roasting over a slow fire somewhere.)
In any case, it's presumably the same as would happen to intentionally aborted fetuses. So it's really no great harm to a fetus to be aborted, besides the actual pain of the procedure, if it's done after the nervous system is sufficiently developed (I think a majority of abortions may occur even before this happens, but I'm not 100% on that).
To finish the story, my argument (which I sort of abandoned mid-formation) maintained that what made murder of an adult worse and justifiably unlawful was that, in addition to having whatever value a soul confers, adults also value their own lives. Fetuses and their ilk do not, so their murder isn't as great a loss. Let God judge those who have abortions. (Of course, then there's the whole problem with infants. Personally, I bite the bullet with Peter Singer on this one and see infanticide as less immoral--but still wrong!--than adulticide.) And so on and so forth.
But this, I think, is a far more promising direction. Once people realize what outlawing abortion actually does, what happens to women who will get abortions illegally, only the most diehard anti-choicers would continue to be in favor of an outright ban. (I mean the fact that women who have been raped don't even have an exemption in SD is just sick.)
There are a number of things I want to say to religious individuals in emphasizing this point on practical consequences. It's between a woman and her God if she chooses to have an abortion. Let the individual make the choice and if you're against abortions, don't have any! Tell your friends not to have any! Feel free to continue discouraging women from making that choice, but in the end let them make the choice.
Unless you're prepared to grant all of the things that come with personhood to children, infants, fetuses, and zygotes, you can't make a binary distinction between persons and non-persons. And if you don't go the all or nothing route, then you admit that while abortion may be like murder, it is not actually murder (the killing of a person in the full sense) and should not be subject to the same restrictions.
This doesn't yet prove that any or all abortions should be legally permissible. But at least it starts to raise questions and doubts. I mean, if you're so damn sure of the truth, why haven't you even drawn the most basic consequences for what you're advocating? Use your goddamned purportedly God-given minds, people!
Really, though, I wish more rightwing Christians took that whole "render unto Caesar what is Caesar's" line from the Bible more seriously, and just stayed the fuck out of politics.