Unrestrained immigration.. legal or illegal lowers wages and wrecks the standard of living for the working class.
This is a documented historical fact.
In 1960, Beef packed was the second highest paying industrial job in the United States
Now it is one of the worst jobs in America.
Immigrants were a big part of making that happen.
"the development of new corporate strategies fundamentally transformed the capital-labor relationship and led to the collapse of industrial unionism in meatpacking. Most important were the industry's development of new technologies, it geographic reorganization of production, and it's ability to locate new cheap non-union sources of labor."
Iowa Beef Production sent recruiters to the poorest parts of Mexico and bused people all way to the American Midwest to undermine unions and lower wages.
Bonus points for not passing any of the savings on to the consumers and pocketing the profits.
sources:
http://migration.ucdavis.edu/rmn/more.php?id=144_0_2_0
http://wox.sagepub.com/content/30/3/327.abstract