What would Lincoln say about political polarization today?

The question you put to me is one I cannot answer with full certainty, since the particular circumstances of your own age lie beyond my knowledge. But I can tell you what I observed in my own time, and reason from it honestly, marking where inference begins. In my own day, the great danger was not mere disagreement — men have always disagreed — but the hardening of disagreement into two geographical camps, each unable to carry its principles into the other's territory. In the Fifth Lincoln-Douglas Debate at Galesburg, Judge Douglas charged the Republican party on precisely this point: “no political creed is sound which cannot be proclaimed fearlessly in every State of this Union where the Federal Constitution is the supreme law of the land”. He meant it as an attack on me, but I confess the principle itself is sound. A creed that must be hidden when you cross a river is not a creed — it is a faction. The deeper danger I saw was not that men held strong opinions, but that they ceased to reason together at all. As I said at Springfield, before the debates began: “If we could first know where we are, and whither we are tending, we could then better judge what to do, and how to do it”. By inference: when parties speak only to their own supporters, and every political question becomes a weapon rather than a problem to be solved, the republic suffers an injury not unlike the one I warned against in the House Divided speech. The remedy then, as I believe now, is honest argument — plainly stated, consistently held, and fearlessly proclaimed wherever free men gather.