To be established: elective abortion unjustly takes the life of an innocent human being.
Premise #1: intentionally killing an innocent human being is morally wrong
Premise #2: elective abortion is the intentional killing of an innocent human being
Conclusion: therefore, elective abortion is morally wrong
Note - we are talking about morally wrong, absolutely wrong. Wrong yesterday, wrong today, wrong tomorrow. Not just in my opinion, If you don't accept premise #1 above, you can stop reading now. Nothing that follows will make any sense. Elective abortion means we are excluding those circumstances where the physical life of the mother is threatened, such as an ectopic pregnancy.
So, the central question is, is the baby in the womb a human being? If it is not, no justification for elective abortion is necessary, but if it is, no justification is adequate.
From a scientific point of view, we can conclude: yes, the unborn baby is human. It has all the characteristics of a human being: 1) it is complete, all the information necessary is present (unlike in the case of the sperm or egg). 2) it is unique, genetically distinct from its mother. 3) it is living, growing, developing, responding to stimuli. 4) "taxonomic-ally", it is human; living things always produce after their kind, so humans beget humans.
Note that these things are all true the moment an egg is fertilized.
How it the unborn different? It is smaller than born humans, It is less developed. It is in a different environment. It is more dependent on its mother than the born human. But none of these disqualify it from being human; a toddler is also smaller, less developed and more dependent, but nobody would say a toddler is not human.
So from a scientific point of view, the unborn is human.
How about from a historical perspective? History tell us that there are terrible consequences when we separate humanity from personhood. The reasons are inevitably used for nefarious purposes. We have, at various times and in various places, dictated that blacks, Indians, women, and Jews are not persons.
From a human rights perspective: if you believe in universal human rights, then you believe all humans have them inherently. The Declaration of Independence called that "self-evident". They are not granted to them by man, by law, by governments. If the preborn is human, it has these rights.
These are the reasons why I say elective abortion is morally wrong.
This raises the question: but what about the rights of the mother? And this is a question we can't just set aside, Though it takes male and female to create a baby, the burden of pregnancy falls completely on the female. We might ask, is this fair? "Fair" does not enter into a situation like this. It is "the way things are", the nature of our species and how we reproduce. This is not because of privilege or social constructs; it is biology. Is it fair that men cannot bear children? Is it fair that birds can fly and humans can't? We recognize these are silly questions just in the asking of them.
The pro-life position therefore necessitates the placing of an enormous burden on the woman who finds herself pregnant against her desires (possibly against her will). Of course, many laws place burdens on us: we are required to pay our taxes, restrict our speed, comply with community standards regarding dress, alcohol consumption, etc. If we were to construct a scale to represent how much of a burden we bear, the pregnant women's score would be very high compared to others: set a value of 100 or 1000 or 1000000 to it, and recognize it is a great burden. But the burden of the child killed through elective abortion must be scored higher yet. It has been deprived of life itself. No matter the desperation of the mother's situation, it is less than that of her child. Even in cases of rape or incest, the child's life should not be taken. It is an innocent party in the matter. And, let's be honest. Rape or incest applies to a very small percentage of elective abortions.