Obama announces immigration overhaul shielding 4 million from deportation

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32. they are already working

so they are already part of the labor pool.
for example: Fields of fruits and vegetables have had to be plowed under when there are stricter rules and sweeps for illegal immigrants.
and" Illegals work forest jobs for less money. Other contractors have to meet their lower costs to get the job. This will put them on equal footing because they won't be able to work for less than minimum wage.

A lot of Americans have been benefitting from the illegal labor pool - household help, elder care, agricultural, etc. They will now have to pay at least minimum wage and meet labor standards.