The bylaws of nearly all unions don't prohibit undocumented immigrants from joining.
However, Jerry Hunter, former general council for the National Labor Relations Board, said unionized construction workers might balk if their unions recruit illegal immigrants.
"Members could start asking themselves, 'Whose interests are you representing?'" Hunter said. "Day laborers or union members who are skilled workers?"
Ultimately, the objective is to hit up some fresh suckers for dues.