I would say Richard M. Nixon, with his infamous 'Southern strategy', which was to pander to racists and bigots and dumbasses of other kinds for their votes. In the South they had all been staunch Democrats since the Civil War until that Party supported the Voting Rights and Civil Rights Acts. He correctly realized that the Republican Party would never win another national election without them. He mistakenly thought the Republican Party could use them in order to further its agenda (looking after the interests of Big Business and very rich individuals) without having to do anything more than pay lip service to their 'cultural issues'. Via the 'evangelicals' (who brought the first wave of Republican 'populists' into Congress in 1994), then the Tea Party, and now MAGAts, "deplorables" (Thank you, Hillary.) have taken over that Party.