Governments in the US subsidize immigration through a bevy of welfare programs. The effect of subsidization is predictable: you get more of what you subsidize. This is true for student loans, ethanol, immigrants, and more.
This has been a hot topic both on Stacker News and wherever else people spend their time. I thought this article laid out the current state of how much migration is costing the American taxpayer.
The article also makes the important point that we aren't just talking about open borders. Anything that receives subsidies will end up exceeding the optimal equilibrium level. That means there are too many migrants living in the US for everyone's own good. Since the migrants are presumably coming because it makes them better off, we can conclude that it is making Americans even more worse off.
There are plenty of relevant caveats to what I just said, but that is the starting point of how to think about the situation economically.