Can the polarization between Democrats and Republicans in the U.S. be reversed?

[Anomalocaris]

No, the people who made the concession were the people who wanted to leave the union and keep slaves.  They did not make the concession.  The concession was taken from them.   The fact that they then waged a century long political war to regain parts of what they conceded and appears to even recently be making progress that cheerier view of history had not allowed to be possible does not reverse the overall balance.

No, you're wrong.  The rapprochement was a result of a concession from the northern Republicans and a failure to secure legislation contrary to southern interests.  Neither were a result of southern Democrats giving up anything.  The intolerance between southern Democrats and northern Republicans was not ended by the Civil War, although the southern Democrats did give up slavery as a result of their loss of the war.   The Civil War continued, just through, in Clausewitz' phrase, other means.

Gradually, though, as the Civil War generation passed from the scene, Democrats and Republicans learned to live with one another. They heeded the words of former House Speaker James Blaine, who in 1880 advised fellow Republicans to “fold up the bloody shirt” and shift the debate to economic issues.

It was not just time, however, that healed partisan wounds. Mutual toleration was established only after the issue of racial equality was removed from the political agenda. Two events were critical in this regard. The first was the infamous Compromise of 1877, which ended the 1876 presidential election dispute and elevated Republican Rutherford B. Hayes to the presidency in exchange for a promise to remove federal troops from the South. The pact effectively ended Reconstruction, which, by stripping away hard-fought federal protections for African Americans, allowed southern Democrats to undo basic democratic rights and consolidate single-party rule. The second event was the failure of Henry Cabot Lodge’s 1890 Federal Elections Bill, which would have allowed federal oversight of congressional elections to ensure the realization of black suffrage. The bill’s failure ended federal efforts to protect African American voting rights in the South, thereby ensuring their demise.

— "How Democracies Die," Steven Levitsky and Daniel Ziblatt.

African American social status and voting rights may seem like the signature cause of the war between the states.   It was not.  It was a convenient rallying point and basis for rallying points.  The real factor whose importance led to acceptance of the war on both sides was ascension of northern industrial interests and a reversal of balance of political and economic power between north and south.  The war prevented the south from taking their toys and going somewhere else, and ensured they have no choice but to remain in a national polity in which northern industrial interests dominated.

The north won.  The fact that the north had to make periferal concessions to keep its winnings does not change the fact that it was a primarily industrial US that emerged and lasted down to this day, and the south got some carrots, knowing the north has the guns and will shoot.

Enlightened winner in conflicts of fundamental interest will disburse many carrots to the loser.   But Winners never won mainly by giving away carrots.  Winners in conflicts of fundamental interest always win by instilling the certainty created by experience that they can and will shoot, and will win when they shoot.  Carrots merely weaks the loser’s desire or resolve to periodically test and validate this certainty.

Durable winnings in fundamental conflict is usually a lot less than the winnings the winner appears to be able to take at the moment of victory. Winners have to give up much of the winnings in order to secure the vital part.  Winners who try to take it all usually crash and burn in short order.  That has always been a key fact in diplomacy, including the part that comes after the other means. This is actually being seen between 2016 and 2018 right here.