I'm no Lincoln scholar but it seems to me that he was (A) a man of the 19th century, not the 21st, and (B) in favor of keeping the union together if he possibly could. Here is a letter of his written in 1855, in which he says this:
Our progress in degeneracy appears to me to be pretty rapid. As a nation, we began by declaring that "all men are created equal." We now practically read it "all men are created equal, except negroes" When the Know-Nothings get control, it will read "all men are created equal, except negroes, and foreigners, and Catholics." When it comes to this I should prefer emigrating to some country where they make no pretence of loving liberty -- to Russia, for instance, where despotism can be taken pure, and without the base alloy of hypocracy [sic].

That seems not so very different from much of what has already been said in this thread.