Which they are.

From The Political Impact of Immigration: Evidence from the United States by Anna Maria Mayda, Giovanni Peri, and Walter Steingress.

From the Abstract:

In this paper we study the impact of immigration to the United States on the vote for the Republican Party by analyzing county-level data on election outcomes between 1990 and 2010. Our main contribution is to separate the effect of high-skilled and low-skilled immigrants, by exploiting the different geography and timing of the inflows of these two groups of immigrants. We find that an increase in the first type of immigrants decreases the share of the Republican vote, while an inflow of the second type increases it. These effects are mainly due to the local impact of immigrants on votes of U.S. citizens and they seem independent of the country of origin of immigrants. We also find that the pro-Republican impact of low-skilled immigrants is stronger in low-skilled and non-urban counties. This is consistent with citizens' political preferences shifting towards the Republican Party in places where low-skilled immigrants are more likely to be perceived as competition in the labor market and for public resources.

It's just one study but consistent with my long held hypothesis. Attitudes towards immigration are shaped more by the affect of immigration on one's quality of life (including economic quality of life than it is by preexisting opinions about immigration policy, attitudes or hypothesized racism, xenophobia, etc.

The party aspect is of less interest except as a proxy for encouragement of immigration as an unalloyed good (Democrats) and a more cautious and utilitarian consideration (Republicans).

If you are wealthy and socially established, it is likely that high-skilled immigrants increase the quality and variety of your professional experience and low-skilled immigrants reduce the cost of semi-luxury items such as construction, landscaping, lawn services, etc. For this elite, immigration is a positive.

For everyone else, those who are financially or professionally precarious, the young starting careers, the poor and unskilled, high-skilled immigrants have a marginal impact on quality of life and unskilled have a detrimental impact.

I suspect that some of the revolt of the masses across the developed world is in part a product of the consequences of the multiculturalism/open borders policies of the 1980s and 1990s in Europe and the US. The wealthy and socially established experienced a marked improvement in quality of life from these policies while everyone else had no benefit or a detrimental experience. It is not about xenophobia and racism - that is just one of the bigotries of the elite towards their own fellow citizens.

Whether you have more or less open boarders is not really the issue. It is more about how many and what are their impacts on whom. Average impacts for the whole system are one thing but individual experiences are another.

If a small elite benefit significantly and everyone else suffer a little and a small number suffer a lot, you can a small average system benefit while the bulk of the population have a decline in quality of life.

I don't know that that is what is happening but that is what I suspect is happening and which the elite are not attuned to, appearing out-of-touch and disrespectful of their fellow citizens. Which they are.