In my country, and I assume in the US, immigrants are useful to the bourgeoisie as highly-vulnerable and highly-exploitable labor. As many say, they do the hard and dirty jobs which other people don't want to do, they can be coerced with threats of deportation. And with the US Republican Party's apparent attempts to reduce offshoring and bring production back onshore, surely powerful sections of the bourgeoisie have an interest in lowering labor costs to compensate.
A popular answer from the mainstream is to simply point to White nationalism and other bigotry. Another answer is that illegal immigrants acts as a scapegoat and a way for their party to signal to citizens that they're doing something to solve their problems. And while I think these are reasonable and plausible suggestions, I want to explore other options before assuming a simple cultural explanation. For example, liberal media tends to frame Middle-Eastern conflict with the Zionist Regime (and even Western support for the regime) as religious conflict or even a racial conflict, rather than colonial imperialism.
Why are you dismissing the scapegoat explanation as “cultural”? The capitalist project is essentially to convince the turkeys (the public, or voters in a democracy) to support Thanksgiving dinner (their own exploitation). To do so, they have to convince the public that someone else, and not the capitalist class, is to blame. Immigrants are a favorite target to play this role, with cost of living, low wages, high housing prices, taxes to fund social services, and more being attributed in whole or in part to foreigners or “invaders” whom the capitalist promises to remove in order to supposedly benefit the public economically.