Actually it wasn't about that. They had no intention of freeing any slaves when the war began. It wasn't until nearly two years into the war that Lincoln issued an order to start freeing slaves.
They fully intended to defeat the South, and then continue business as usual, including keeping the slaves exactly where they were.
You may not be aware of this, but those slaves accounted for 73% of all the money the Federal government received. The Republicans of that era were Tax and Spend Liberals, and they wanted those federal revenues to pay for their big government projects. They wanted to build canals, and railroads, and roads, and everything that modern liberals want to spend tax dollars on.
It was quite corrupt, and there were a lot of kickbacks going on for all their government funded projects.
People really don't want to look at this history too closely. It's actually very ugly, and nobody comes out of it looking good. There is a reason the Grant Administration was considered the most corrupt administration in history, though people believe that Grant himself was not actually corrupt.
It's all those snake bastards that Lincoln put into government that became the corrupt influence peddlers. This is how Washington DC developed it's corrupt crony capitalist culture.
Except for the fact that the pro slavery argument was coming from the Democrats and they anti slavery position was held by the Republicans.
This massive disagreement became a national political conflict, and then a civil war.