I've been thinking about this parallel for months, and it is obvious and logical. Slavery and Abortion. In the case of slavery, the argument was made that Negroes were not people, or ere not human, and thus they deserved no constitutional rights. Negroes could be treated as anyone wished, as they were the equivalent of draft animals. Nowadays, unborn children are not considered to be people, and thus, have no rights. They they can be treated however one wishes, whether that be embryonic stem cell research, or simply being dismembered in their mother's womb.
Having slaves helped the economy. It meant cheap labor, as there were no official standards of living for slaves. Plantations could garner more profit.The ethical argument could be made tat it meant that there would be no children forced to work in the fields. (Negro children aren't persons, so it's okay if they work to death.)
In the same manner, the abortion industry garners millions each year in profit, which it can then invest in companies, and even to charity. The ethical argument could be made that it saves children from a life of abuse, neglect, and poverty.
Unborn children are considered by many to be nothing more than a parasite, or a part of the mother's body like an organ. In that sense, it would be her property, since her body is hers. She can do whatever she wants with her body, which includes ending the child's life at early term, late term, or for Obama, even after the child is born.
Slaves were considered by most to be noting more than draft animals, not sentient or capable of critical thought or choice. Being owned by the master of the plantation, in that sense, they were his property, since his belongings are his. He could do whatever he wanted to do with them, which includes whipping, branding, starving, mutilating, and if he deems it to be good for him, killing the slaves or their children. (Incidentally, sometimes even by forced miscarriage.)
Also, if we get rid of slavery, people will still have slaves because the conditions that necessitated slavery will still exist. Outlawing it will make it unsafe ad cruel and dangerous for slave and master alike. At least if slavery is legal, the government can put legal restrictions on it.