There is a very well-written open letter to Barack Obama from a Princeton senior on the National Review website. The author, Sherif Girgis, has the audacity of hope that Barack Obama will recognize the right to life of all Americans, including the unborn.
Girgis quotes the Declaration of Independence to the effect that "all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life." This is a standard appeal in the pro-life movement. Although I agree with the natural law argument against abortion, I don't find the Declaration as straightforward about the right to life as others do. The reason is that we have two competing natural right traditions, and the Declaration is not clear to which tradition it appeals.
One tradition is the "classical natural right" tradition that originates in Plato and is developed by Aristotle, the Stoics, St. Thomas Aquinas, and some modern thinkers like Hooker and Thomas Paine (an excellent, straightforward introduction to classical natural law can be found in Plato's Modern Enemies and the Theory of Natural Law by John Wild.) This tradition finds its basis in nature and what common experience reveals about nature. Briefly, nature reveals itself to be an intelligible organization of dynamic beings. These beings have definite natures that are essentially dynamic and embody specific existential tendencies that they strive to fulfill. A puppy, for example, has a doglike nature and will naturally strive to fulfill its doggy nature. Similarly, man has a definite human nature and he has a natural inclination to fulfill himself as man. The specific difference of man, however, is his rationality; he not only has an end that fulfills him, but he can also rationally perceive that end. This gives man a freedom not granted to other animals. The dog is not aware of his own dogginess and fulfills his nature as a dog as a matter of course. Man, however, is aware of his own nature and it is his nature to fulfill himself through the rationality that is specific to him. He fulfills himself by coming to know his end and the means necessary to acheive it. The natural law refers to the principles that man comes to know as necessary to fulfill his nature. It is called natural both because it refers to the essential tendencies of man's nature and also because man can know it by his natural reason (i.e. without the necessity of revelation.)
One of the things that natural reason reveals is that man is by nature a social animal. It is his nature to live in communion with other men and respect them. This is the basis of justice in natural law and is the foundation of the pro-life natural law argument.
The Declaration of Independence, however, is ambiguous about the source of the rights it asserts. It writes that man is "endowed by his Creator" with certain unalienable rights. Now it is obvious that man is endowed with eyes, ears, hands, and a mind, whether we wish to say the endowment comes from "nature" or from "nature's Creator." So much is manifest from common experience. The classical natural law tradition of Plato and Aristotle concludes the natural law from this manifest human nature. The natural law follows on the human nature that we can all see and analyze for ourselves. The natural law, for the classical tradition, is not an endowment to man but a conclusion from man's nature. This is a very important difference, because man can lose certain endowments (such as his eyes or his ears or even his mind) and still be a man. And if he is still a man, the natural law follows from his nature. The natural law is in that sense unalienable - obviously a very important point for the pro-life cause. But if natural rights are an endowment, then man can lose that endowment and still be man; natural rights do not pertain to man as such. Natural rights become "alienable" even though the Declaration insists they are not. In fact, the Declaration seems to "protest too much." Plato and Aristotle had no need to insist that natural law is unalienable because it obviously is: It follows on man's manifest nature. By calling rights an "endowment", the Declaration puts them on the same footing as man's eyes, ears, hands and mind. Yet man does not obviously have rights the way he obviously has eyes, ears, hands and a mind.
This brings us to the alternative natural law tradition, the tradition that starts with Thomas Hobbes, goes through John Locke, and has become the dominant natural law tradition of today. This tradition views classical philosophy and its "natures" with suspicion and attempts to find an alternative foundation for natural right. Hobbes thought he found that foundation in his doctrine of the "state of nature." The "state of nature" isn't really about some primordial human past; it is about the basic human condition here and now. Man is clearly not equal in his endowments of virtue, talent or fortune; some men are born courageous and temperate and some are not; some are born into noble families and others are not. But even the lowliest servant in the castle is capable of poisoning the King. (Michael Corleone: "If there is one thing we have learned in this life, one thing we can be sure of, it is that you can kill anyone.") The equality of men, therefore, can be found in their vulnerability to violent death. The only way men can live, and that includes the King, is if they refrain from killing each other. This is what Hobbes means by saying that the state of nature is the "war of all against all." The "right to life" in this tradition is the right to defend yourself in the state of nature.
Why should anyone respect this right? Because they are as vulnerable as you are. Only if we respect each other's right to life can we live. And the King must respect the servant's right to life because the King is as mortal as the servant.
We can see that what is really behind the "right to life" in the modern tradition is the threat of violence. I have a right to life because I can assert that right and threaten your life if you choose to ignore me. Rights have no basis in nature; their basis is in the assertion of the right itself. The Declaration asserts that "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights..." Suppose that you do not find that endowment self-evident, as apparently George III did not? That is beside the point. The Declaration does not ask for rational assent to its assertions of right, let alone rational assent based on a commonly known human nature. It asserts the rights and threatens violence if they are not acknowledged (specifically, the colonists will "throw off" the government if its rights are not respected.) What matters is that we find the rights self-evident, even if you do not, and we back up our assertions with guns rather than dialectical arguments. The self-evidence the Declaration asserts is subjective and particular, not objective and universal.
Our rights are unalienable, then, because our capacity for violence is unalienable. The King can't take away our right to life because he can't eliminate the vulnerability of his own mortal nature (or, by extension, the mortal nature of his imperial army.) But what about those who can't threaten the King with violence, say, the unborn and the elderly? The King is not worried about a revolution of infants or codgers. The unfortunate implication is that since they cannot assert a right to life, these folks have no right to life. This is why euthanasia and abortion are permanent temptations for our Republic.
This is some of the ambiguity involved in appealing to the Declaration of Independence in the pro-life cause. It is also, I think, why the argument from the Declaration does not get as much traction as pro-lifers suppose it will. Girgis, for example, asks some very pointed questions of Obama: Is the heart stilled in an abortion a human heart? Are the limbs torn apart human limbs? Girgis is arguing here powerfully but squarely from the classic natural law tradition; he is arguing from the nature of that which is killed in abortion. But the Declaration he earlier cites does not seem to base right in this tradition. It is possible for one to agree with Girgis's argument about the nature of abortion yet disagree that the Declaration demands that the rights of the unborn be respected. The rights in the Declaration are based on the assertion of right, not derived from a commonly known human nature. Since the unborn and the elderly cannot effectively assert their right to life, they have no right to life, even if they are human beings.
What needs to happen for the pro-life cause to succeed is for people to understand that classic natural law transcends such human documents as the Declaration of Independence. The Declaration does not demand that we respect the rights of the unborn, but the natural law we can know through common experience does. Classic natural law is not in conflict with the Declaration because the rights asserted in the Declaration truly are rights; but they are not a complete account of rights nor a complete account of the foundation of rights. It is true that I have a right to life because I can kill you. But I also have a further right to life, based on my human nature, that calls for respect even if I can't kill you. The Declaration is fulfilled in the classic natural law tradition, but it is not really based in that tradition.