“The GOP took the country dominantly for one reason only:”
Lincoln was elected with only 39% of the vote. He was a sectional candidate in a four way race.
Post Civil War the South was occupied territory with generals ruling over military districts. ‘Dominating’ politics during Reconstruction was as easy for the GOP as for the soviets when they occupied eastern Europe. It would have been impossible for them not to.
Historians have cut the numbers seven ways from Sunday. It ultimately came down to an anti-slavery vote, really by about 3:1. Only one of the four parties stood for slavery, the southern Dems.