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That's not the point! If we do what some would like, and eliminate or
severely curtail the flow of "illegals" into this country, how do you think
we'd replace the labor that they're willing to do? Or, to put it another
way, your 16 year old kid says "Dad, I just got a job. I'll be bent over in
the sun for 10 hours a day, swinging a very sharp knife at brocolli stems
right near my ankles. The crops are sprayed with some sort of chemical a few
days before they're picked, but the guy at the interview said the stuff was
safe to touch. It's four bucks an hour".

You OK with this scenario?

This is BS. Four things would happen. For some of the jobs, Americans
would fill them at the same wage. For others, Americans would fill them
for more money (and the costs of those would go up). Some would be to a
greater or lesser degree mechanized and the rest would disappear.
Probably the latter would be mechanized.