Trump is allegedly coming out with a new immigration plan to try to unify Republicans.
If you want to understand the problem, it begins with the unintended consequences of racism (the Southern, Conservative legislators' belief that a "family" exemption in the 1964 Act would keep the US mostly white and mostly northern European) and moves on to the rise of conservative,Barry Goldwater, which has left the Republican part y today as the so-called "party of white retrenchment" (i.e., Trumpism)
https://www.politico.com/magazine/story/2016/08/immigration-1965-law-donald-trump-gop-214179
Great summary ^^^^. Interesting in that it doesnt account for the impact of voter suppression and gerrymandering.