I see a major difference with this comparison. I don't know if Lincoln can be said to have been in favor of slavery at any point in his life. Perhaps he was, but I've not heard or read that. What his position on the subject was did not reflect his personal opinion on slave owning, but on what should be done about slavery in the states where it existed. Even in Chapman's piece, it implies that Lincoln felt the institution would die out on its own without gov't forcing the issue. Thus, there's nothing to suggest that he ever hoped to support it or defend it.
Obama, on the other hand, had stated his support for it, then changed and now has changed back to where everyone, on both sides of the political divide, knew he was the whole time.