Where did the idea come from that we can't find and deport 12 million illegals?
My experience in law enforcement tells me this is not the case.

As a cop, I regularly came into contact with illegals. Sometimes as many as 10-15 in the course of a week.
Not every contact was in the course of making an arrest, but many were. 
I lost count in a hurry of how many illegal immigrants I arrested or ticketed for traffic offenses/DUI.
This was in a rural Georgia county with a not particularly high concentration of illegals.

Now, when our agency tried to get INS to deport these people, they basically told us to call them back when we had a couple of busloads, and they might be interested.

Statistics show there are about 800,000 sworn law enforcement officers in America.
Over 700,000 of these are at the state and local level.

Based on my experience, I think it's safe to assume every state/local officer has at least one contact per month with an illegal. Simple math says that equals 8.4 million contacts in a year. That equals two thirds of our estimated illegal population!

The line that we can't find and deport these illegals is baloney!  We just don't have the will to do it.