I do think it's one of those subjects that reasonable people can disagree about. Personally I'm quite pro-immigration and think there should be fairly straightforward and transparent paths to citizenship for people who come here. I think cultural homogeneity is a bad thing as it tends to be self-reinforcing with homogeneity leading to increasing conformism. I think this country would be a much less interesting place without the recent immigration boom, which followed decades of historically low levels of immigration.
But you can be a good liberal or progressive and have different views about that. Many peoples' opposition to immigration is rooted in deeply illiberal beliefs and there are some obvious liberal positions on how to deal humanely with immigrants, legal and illegal, once they're here, but I don't think there's One Liberal Viewpoint on the appropriate scope and composition of the US immigrant population.
