Republicans used to be the urban pro-industrial party, democrats were the party of rural farmers.

Urban industrialists didn’t directly rely on slave labor so they were more open to abolitionists. Rural farmers were heavily dependent on slave labor so they were very pro slavery.

It’s also why the democratic party has always tried to get universal health insurance passed. The party is made up of farmers with law degrees, so the local Doctor, the only other educated man who it was appropriate to socialize with would come around a lot— so they were constantly hearing stories about how their friend had to accept chickens or food as payment instead of money.