Are there enough Americans who are willing to do the jobs that the foreign/illegal workers currently do? I know that in San Diego, there are many foreign/illegal workers performing landscaping because no one else wants to do these jobs at the wages they are getting paid.
The only way to get local workers to do the same jobs is to raise the wage... but if they raise the price, no one is willing to pay.
The theory is that if we stem the flow of illegals, there will be less labor available. Less people means that wages will go up for currently low-paying jobs, and if wages go up these jobs will become more desirable.
Also, from what I've read on Trump's transition site, a larger emphasis will be placed on legal immigration and allowing demand for specific skills to dictate who gets in.
Poe's law strikes again? :p