Leave Slavery Up to the States

We contrast slavery with killing babies and find that politicians tend to go for pragmatism or principle. Shall we have slavery or abortion where "everyone wants it?" Do we leave this up to the individual states, as the Republican presidential candidate puts it? If the goal is to "save the nation," will this accomplish the goal? That didn't work for slavery, and it is doubtful it will work for abortion. How would you save this nation?