Many, if not most, principled objections to legaling abortion are not based on the fetus possessing a soul, although my personal belief is that it does. The primary objection is that abortion deliberately kills a living human being without just cause, which is the definition of murder by virtually all civilized people without regard to religious belief.
It is undeniable truth that the fetus is a living human being, check the unique DNA signature. It is also undeniable that the fetus is alive from the moment of conception, if not it couldn't multiply cells and grow in size and complexity. It is also a unique organism, it doesn't share or exchange blood or other tissue with the mother. It should be, and is, obvious to any reasonable person without an agenda which neccessitates dehumanizing the fetus that it is nothing less than a living human being, although I can agree there is no way to prove the presense of a soul either in an adult human of a fetal stage human.
The act of deliberately killing an unborn human being without just cause meets the historical definition of murder, whether or not the federal judiciary defines it as such.