[DiogenesLamp]

Why do you think the two were in opposition?

They are never in opposition. People will always make up some moral justification for their own economic interests.

What do you think BLM is?

[BroJoeK]

"People will always make up some moral justification for their own economic interests."

But in the case of abolitionism, the moral arguments against slavery came first, decades earlier.
Those are what caused Northerners to outlaw slavery in their own states and in western territories.
But Northerners were tolerant of slavery in the South and in their national political parties.

What united a majority of Northerners behind Republican antislavery ideas was the perceived threat of explanded slavery after the SCOTUS 1857 Dred Scott ruling.
Their opposition was both moral & Constitutional before it was economic.
The combination is what turned many Democrat states in 1856 to Republicans in 1860.