Catton cites Life of Alexander H. Stephens, by R.M. Johnston and W.H. Browne as his source for the Stephens quotes. Where they got it I can't say. That kind of prophesying wouldn't have been well received among his fellow southerners, I shouldn't think. As a politician he would have been reluctant to offend them with such unpopular words.
Oh, drat. I thought I accidentally deleted my first attempt at a reply to you so I tried to recreate it. Now I see that I got it posted. Feel free to choose which wording you prefer.