Of course, the Republican antislavery movement never abandoned the moral argument against slavery, but they fused it to a material politics. They made the case successfully that what they called “the Slave Power” threatened not just the moral principles but the actual livelihoods of Northern voters.

William Seward, who’s probably the most important pre-Lincoln Republican political figure, went through the math and said slaveholders compose not “one hundredth part” of the United States — about 300,000 slaveholders in a country of 30 million. He didn’t do a Bernie Sanders accent, but he made the point: this was an antebellum 1 percent that dominated the federal government, that set its land policy, its infrastructure policy, its trade policy, all sorts of economic structures that favored the slaveholding regime and undermined the material well-being of northerners from Michigan to Pennsylvania to Maine.

The Republicans were able to make that case and connect it to the criminality, the sin of slavery. It was a really powerful moral-material fusion that inspired not just the handful of abolitionists who were already there, but masses of small farmers, workers, and ordinary Northerners. [...]

"[I]n some ways I think that question [Who freed the slaves?] actually was framed by people who want to produce a simplistic answer. [...] I think I would say the antislavery movement freed the slaves. That would include antislavery politicians; antislavery voters; the Union Army, which became an armed wing of that movement; and, of course, the slaves themselves, who both took part in the Union Army and destabilized the system of slavery on the ground during the war.

I think a lot of the other answers are actually pretty ahistorical. I think “Abraham Lincoln” isn’t a historical answer. [...] Same thing for people who say the slaves freed themselves. [...] Not to be too polemical, but to say that the slaves freed themselves entirely is to say that all of the other enslaved people that did not free themselves at other points in history, and in other countries, etcetera, failed to free themselves every day that they were enslaved.

https://jacobinmag.com/2020/07/antislavery-mass-politics-matt-karp-vast-majority