Hummmmm. Funny thing is that in 1856, the first year a Republican ran for presidenton an anti slavery expansion platform, the "Know Nothing" Party's presidential candidate had the vast majority of his support in the South.
Hummmm? Maybe you want to check your premises. 
I stand by my premises and you really should have thought that through before posting it. The Know Nothings were not a political party. They were a secret political organization who ran their candidates under the banners of other political parties. They did try unsuccessfully to come out of the closet as the American party in that election, but most of their members had already joined the Republicans by that time. The relatively few Know Nothings who were from the South of course did not join the Republicans and account for most of the votes for that reason.