http://michellemalkin.com/2010/11/30/dream-act-nightmare-a-massive-open-borders-entitlement-program/
Short version from the People's Cube:

Which is why Democrats who were beholden to union votes weren't necessarily pro-illegal alien.
The rank & file working Democrat voter (the union guy) - is not pro-illegal alien. But the lame duck congress doesn't have to answer to him. Nor does it answer to the rest of the population that opposes granting citizenship to what amounts to underground invaders.*
Of course, if the current congressional leadership simply takes via taxes from Citizen A and Legal Resident Alien-Who-Worked-His-Ass-Off B and gives via handouts to Illegal Alien C, D, E, F, and G, then that congressman now has C, D, E, F, and G voting for him/his party in perpetuety... and as for Citizen A and Legal Resident Alien B? A bureaucrat with the swipe of a pen has told them that their country isn't theirs anymore - and they don't have the votes to change it anymore.
*I have no idea if that was a 50s sci-fi movie, but it should be.