If you are a legal Mexican/American living and working in America, the last thing you would want to see is 12,000,000 illegals made legal.
The big risk that all Mexican/Americans face is loss of their jobs, because of the job market dilution that would occur with 12 million more workers after the same jobs.
Couple this with 5 or 6 million more illegals heading to America to get in on the offering and you have 40,000,000 very unhappy Mexicans.