The driving forces behind three decades of de facto non-enforcement of federal immigration law were largely the interests of elites across the political spectrum.
Employers in agriculture, construction, the hospitality industry, landscaping, and food processing wanted access to cheap, industrious foreign national laborers. So do the well-off households of the American Southwest, whose current aristocratic reliance on immigrant nannies, housekeepers, gardeners, and handymen rivals something out of Downton Abbey. Such facts were why Cesar Chavez and the United Farm Workers were known once to have patrolled the border to keep out illegal aliens and thus not depress wages of American workers and thereby ruin their own efforts at unionization. Large employers like open borders; entry-level and poor American workers obviously do not.