If you cite statistics indicating that Illegals commit proportionally more crime (in certain categories), they will counter with, "You aren't accounting for the fact that these 'migrants' are poorer, younger, more male, and hence shouldn't be compared with Americans at large!"
They don't seem to be able to wrap their minds around the concept that they shouldn't be here, committing crimes, at all!"
Don't forget that they can also always fall back on truly crazy arguments like "No one is illegal on 'Stolen Land!'", "We owe them reparations!", etc.
Regards,
Lefties are insane...simple as that.