MLK was a Republican.
The Republican party in 1861 was a coalition of disparate elements. Formed only 7 years earlier, it contained men who had been Whigs, Anti-Slavery Democrats, Free-Soilers, Know-Nothings, and Abolitionists. By the outbreak of the war, these fragments had coalesced into 3 basic factions: conservatives, moderates, and radicals. The most aggressive and, eventually, most influential of the three was the Radical Republican faction. All Republicans were against slavery, but this group was the most "radical", in its opposition to the "peculiar institution."