This has always been the biggest question for me. I spent a few hours one day researching on the web. I tried to find reliable sources and those with juxtaposed policy positions. What I discovered is that undocumented immigration either has a small benefit, net zero or small cost. It's not huge one way or the other.
I haven't read anything about it, but I do worry about what the long-term effects would be of flooding the country with new poor people via some new guest worker or open borders policy. I'm reliably left of center on social issues, but I also would like to preserve our high standard of living. I would think at some point immigration would reach critical mass and would start overtaxing social services, schools, hospitals, etc. I would also think it would greatly increase the supply of cheap labor, which would lower wages among the poorest people and drive them further into poverty. I worry now about public schools. I have seen in my own neighborhood public school that more than 50% of the kids are Hispanic (and I live in a white-ass neighborhood). I don't know for sure, but I think it's the presence of multifamily housing. I see that doing nothing but increasing here. I'm guessing, but wouldn't a higher ratio of multifamily to single family tend to decrease the property tax base and the source of school funding?