What they’re revealing is their own racism. I have witnessed this myself: they are determined to refuse whiteness to Hispanics tout court. It’s not so much a liberal phenomenon as it is a Protestant phenomenon. Of course liberalism and Protestantism are pretty much the same thing.
If you mean the classical liberalism of the founding fathers as being Protestant, you are correct. If you mean that Protestant Christians are progtards, you ignore that evangelical Protestants are the most right wing religious group in the United States. Meanwhile the cafeteria Catholics here in the northeast ain’t exactly right wingers or devotees of the founding principles of this nation. Of course cafeteria Catholics produce agnostic and atheist children and “devout Catholics” are a tiny minority as well.