there was never a northern president committed to abolition. lincoln only freed the slaves because it was politically strategic. there is a lot of his writing to support this. slaves joined the union army, against his wishes, and that actually helped them win the war. so then he became an abolitionist, but he didn't rush into it. most northerners were trying to protect the union, not end slavery, because their economic system was very profitable due to southern cotton production.

just read foner if you want something easy or read black reconstruction in america by dubois if you want the key text.