Thanks to Ta-Nehisi Coates and others, much of the educated discourse around our current political dysfunction recognizes that the 1965 Voting Rights Act more or less created the situation we find ourselves in.
Prior to 1965, the overwhelming majority of Black Americans lived in the South where they were not allowed to vote. The Roosevelt Realignment created our two 20th Century political parties. Both were the parties of white people.
Black people joined the American body politic all at once in 1965, and their votes went to the Democratic Party that gave them the right to vote. The two parties were now heterogeneous with regard to race: one remained all white, one was mixed. The Republican Party now faced an overwhelming temptation to run as the party of white people. The Southern Strategy was obvious. Not to have pursued it would have been political malpractice.
This much is understood.
But another group got the right to vote in 1965: poor Southern Whites. As a law review article points out, they had been disenfranchised right along with Southern Blacks:
For the Framers of disfranchisement were typically the most conservative, large landowning, wealthy faction of the Democratic Party, who were also seeking to entrench their partisan power and fend off challenges from Republicans, Populists, and other third parties, as well as from the more populist wings of the Democratic Party. While pledging not to disfranchise any whites, they advocated provisions that would remove the less educated, less organized, more impoverished whites from the electorate as well--and that would ensure one-party, Democratic rule, which is precisely what happened from this moment forward through most of the 20th Century in the South.
The above passage cites the following footnote:
Kousser's brilliant book documents this thesis in superb detail. See Kousser, Southern Politics at 238-265. Studies of individual states have concluded that, in some, more whites than blacks turned out to have been disfranchised after grandfather provisions were eliminated or held unconstitutional . See, eg, Malcolm Cook Mcmillan,...
The case that eliminated grandfather clauses was Guinn v. United States, 238 U.S. 347 (1915).
The law review article is Pildes, Richard H., Democracy, Anti-Democracy, and the Canon. Constitutional Commentary, Vol. 17, 2000. Available at SSRN: https://ssrn.com/abstract=224731 or http://dx.doi.org/10.2139/ssrn.224731
The Kousser Book is
J. Morgan. Kousser, The Shaping of Southern Politics: Suffrage Restriction and the Establishment of the One-Party South, 1880-1910, Yale University Press, 1974.