Perhaps some those illegals are descendants of American citizens.
https://en.wikipedia.org/wiki/Mexican_Repatriation
400,000 to 2,000,000 "Mexicans" (including their American-born children, technically US citizens) living in territories annexed by the US were "repatriated" (put into buses and sent to Mexico). In other words, this is ethnic cleansing.
Some of them owned land and properties, and were forced to abandon them with no compensation.
Racists don't give a shit about legal immigration. If all those people of Mexican descent were legal immigrants, they would still find a way to discriminate them. It's just the excuse of the day.