Perhaps your comparison has no validity. A lot of people think abortion is murder. Slavery was a "peculiar institution" which didn't include murder. It was a social compact were the slaves were given lifetime socialism in return for a life lived in indentured servitude. Sucks but hardly murder. You can try to call that murder but it was status quo for 300 years prior to 1860.
Chattel slavery and abortion are both evils. Both were, or are, in operation in this country, under the color of “law.”
The analogy fits fine.