(I'll preface this by noting that I'm Canadian and will never vote in an American election)

One of the reasons that immigration and green cards has always been such a hot-button political issue in the US is that first and second generation immigrants tend to overwhelmingly vote Democrat. You end up with this awkward feedback loop where D's are objectively politically motivated to increase immigration, while for R's it's the opposite. Perhaps I'm oversimplifying, that's just how it was explained to me once.