It's kinda misleading to say the civil war was
actually about political and economic differences rather than slavery if by and large those political and economic difference stemmed from slavery. If we want to get realpolitik about it we could argue that the elite in the South probably didn't believe slavery was morally righteous but were only looking out for their self interest. That doesn't mean the war wasn't fought to uphold slavery, and it doesn't make the cause of the Confederacy any more legitimate.
I can buy that states' rights were a factor, but it certainly appears that the right to own slaves was the biggest one. No, it's not as simple as the great and noble Abraham Lincoln swooping in to free the slaves, but it seems pretty dubious to call the Civil War "not about slavery" when abolishing slavery in the South would instantly cause wealth equal to half of the Confederate GDP to vanish. I'm sure the average soldier in the Confederate militaries felt he was defending his home from invaders rather than defending slavery, but that doesn't change the fact that the institution of slavery underpinned the entire economy of the South and the war was fought to protect that institution. The fact that the war wasn't fought by the North to
free the slaves does not mean that the South didn't fight to uphold slavery.
The agricultural economy in the south relied on slave labour while the northern economy became more industrialized and grew through immigration and the higher value of industry. The North became more and more powerful economically and thus politically, and Southerners became afraid of the idea that eventually the federal power centered in the North would be used to abolish slavery which would destroy the South's economy. The South was beginning to industrialize, but they were so far behind the North and with a smaller population (and far less immigration) had no chance of competing with the North's economy by industrializing.
It can't be stated enough how crucial slavery was to the South's economy, it's true that only a small percentage of wealthy people owned any slaves, and even fewer owned dozens or hundreds of slaves, but the wealth tied up in the slave trade and the agriculture which was made economical through slave labour underpinned the entire economy. Estimates of the value of slaves at the time is around 3.5-4 billion dollars in 1860 money, which is around 80 billion dollars today. We also have to consider the massive increases in real wealth since then. The most important number is that in 1860,
the value of slaves represented about half of the South's GDP. Imagine if Obama were to go on TV tomorrow and call for the destruction of
$7.5 trillion in property, which was 50% of GDP in 2011.