How Do Immigrants Fit into the U.S. Labor Force?

During economic hard times immigrants are often blamed for taking jobs away from U.S.-born citizens. This recession is no different in that regard. The many incendiary comments aimed at immigrants, especially those here illegally, bandied about the GOP primary reflect that as well.
http://www.brookings.edu/up-front/posts/2012/03/15-immigrants-labor-singer