Any civil war is going to have fuzzy boundaries and rough edges, but it was relatively easy. The two sections spent the previous decade plus in a spiral of increasing alienation which worked a substantial political transformation in both. To both sides, their opposite number had proved the case of at least some of the radicals at home that the enemy meant to impose their social system on the other and must be resisted at all hazards lest their ways of life (slavery and free labor) vanish.

But there are dissenters and that dissent sometimes has notable geographic concentrations. That's not true for black Americans, who realize that whatever the whites think the war has revolutionary potential. So as a practical matter, the South starts out with a fifth column about four million strong. Nothing comparable to that exists in the North. There are also southern whites who oppose secession. Many of them come over when the deed is done, but significant rumps do not and engage in anti-Confederate operations where possible. This took place largely in upcountry areas with few slaves and plantations, which always had a degree of political distance from the governing elites or their states. The most famous example is West Virginia, but a similar movement occurred in east Tennessee with less success.

In the North, opposition is largely in the form of the Peace Democrats. The Democrats are the opposition party at the time, but they had a serious wing committed to continuing the war to its end. (Though they were not at all committed to emancipation.) Unsurprisingly, they're the War Democrats. The Peace side was straight-up antiwar and wanted to let the South. I'm less familiar with their geography, if there was much, but the ones usually mentioned are from states bordering on the slave states.