Employer sanctions don't get undocumented immigrants documented. Legalization of the currently undocumented already here and granting of work authorization to all who want does.
Also, if your main goal is to get workers documented, why not argue that unions be given the right to sponsor immigrants for permanent residency? Currently, spouses, parents, etc. can sponsor immigrants for permanent residency for family unification purposes, and bosses can also sponsor immigrants for employment purposes. Unions ought to have the right to sponsor immigrants for organizing purposes.
-- Yoshie <http://montages.blogspot.com/> <http://mrzine.org> <http://monthlyreview.org/>