This question suffers from the grave ambiguity of defining “centrism” at that or any other time. One of the key challenges leading up to the ACW was that the issue of slavery was, in a fundamental way, binary. But even at the time political voices tried to stake out compromise positions. The Republican Party Platform of 1860, which was the thing that drove the South into rebellion, did not call for the abolition of slavery. It merely called for the exclusion of slavery from the Western Territories. In the febrile pre-war political atmosphere of the South, this was interpreted to be the first step on the “slippery slope” to abolition, as at some distant future point Congress would be dominated by free states.

So you are left with the puzzle, perhaps the Republicans were the centrists and the Southerners the extremists? What realistic compromise would have reassured the South that the “peculiar institution” was not destined to be ground under by the march of progress? Legalizing slavery everywhere in the US and the North adopting Southern values?

As the war progressed, the political currents of the North changed remarkably. Had the conflict ended by mid-1862, there is a real chance it would not have resulted in abolition. Lincoln had taken pains to downplay abolition to that point, in order to reassure the border states and leave open the path to a hoped-for “reconciliation” with the South. The Emancipation Proclamation was one of the most remarkable jolts to the Northern political consciousness of the 19th Century. In addition to its historic righteousness, it was one of the most brilliant political and diplomatic maneuvers of all time. But it was hugely unpopular with many segments of the North. There were bloody anti-black riots in several northern states, and it gave rise to the “Copperhead” wing of the Democratic Party that sought peace with the South in exchange for abandonment of abolition.

It can be argue that not until the 1864 election was the median Northern voter in favor of abolition and racial equality. The Democratic campaign of that year promoted fear of African American voting rights and intermarriage as reasons to vote against Lincoln. Many Democratic politicians called for a negotiated end of the war that would reunite the country without the end of slavery. But therein lies the rub: by 1864, after all the loss of life, the South would accept no position short of full Confederate independence. There was no realistic basis for compromise.

The fact the Civil War was about slavery was not obvious to many participants until it was well underway. Many ordinary Northerners would not have cited it as their primary reason for fighting in 1861. But by 1865 its centrality was obvious to everyone. And the impossibility of compromise on this issue was the reason the war went on to kill 2% of the population. There was no realistic path to “centrism”.