The power thus conferred on the South was reinforced by other factors. One was ability. Unlike senators from other sections, southern senators, [William S. White, the Senate's most prominent chronicler] wrote, "had no chance of getting a serious nomination for the Presidency, and they knew it." And because in the South United States Senator was therefore the highest title at which political men could realistically aim, that title attracted men of a very high caliber, so that many southern senators were exceptional individuals, of great personal force and talent. (emphasis in original) pp 90-91

This is deeply insightful.