I doubt that Romney would propose sending blacks back to Africa. Despite all the conscious complaints black people have with living in the U.S., their subconscious mind teaches them that this is way better than living in any country in Africa. People act on their subconscious, not their conscious, perceptions. That's why alcoholics have such trouble giving up drinking.
If Romney proposed free passage back to Africa, he'd be risking assassination, because most of the blacks wouldn't want to go, and they would be in high dudgeon at the thought that some might "shame" themselves and go. The emotions triggering this response are similar to those triggering TRUE homophobia -- the notion that if some others like oneself choose that route that oneself will somehow get pulled into it as into an astronomical singularity. (Pardon my political correctness here.)
Smith also proposed that the fed liberate all the slaves by buying them -- which would have been a lot cheaper than the Civil War, but that war was really not about slavery, so even if Smith had been elected, and the slaves freed, the war would have happened anyway.
I do note that the Church of Christ, Temple Lot in Independence claims that one of the reasons the Mormons were driven out of Missouri was that in the early months of settlement in Independence, the church published an advertisement welcoming "free men of color" to come and settle with them. That was probably the primary instigating factor in the church's gradual adoption of racism, just as in its unadoption of polygamy -- survival in a hostile environment.
But that war was REALLY about slavery. I can't believe it's the XXI Century and some people are still saying this. Slavery, racism, and white supremacy were behind the fighting in Kansas and the central causes of the Civil War. Without racism, there would be no slavery and without slaves the agricultural economy of the southern states would collapse. The emotional need for white supremacy prevented proposed emancipation schemes. Southerners like to say this now to make themselves feel better but if they go back and read what was actually said by their ancestors they will find out for themselves.