1) They use between $100-$200 BILLION more in social services than they pay in taxes. We have to cover that, which means we have less money to take care of ourselves.
2) They increase unemployment, 20 million criminal immigrants are taking a lot of jobs American citizens would normally be doing. Don't buy into the "only do jobs Americans won't" crap. It's a pathetic lie. They do jobs that Americans can't afford to do at the PAY RATE they do.
3) Force down wages. A work force is a commodity, just like everything else. The more there is of it, the less valuable it becomes.
4) Crime rate. Illegal immigrants are significantly more likely to engage in crime than a citizen or legal resident. 1 in 4 non-immigration related criminals in federal custody were in the US illegally. These crimes are a huge drain on our society, and cost us dearly.