Both Democrats and Republicans are wanting the Hispanic vote. Allowing 12 million new citizens would be a tremendous benefit to who ever allows it. Plus the millions of Hispanics that have family amoung those 12 million. Plus lets be honest, there are those who want as many Mexicans to come here as possible, as they believe the Southern border states are really Mexico's and are hoping to get enough Mexicans in to control those states politically, if not forcibly.
Let's not also forget, we still have millions of Americans out of work. Allowing 12 million more US citizens will only put them in direct competition with them. It will also swell our welfare rolls that's already bursting at the seams.
I personally believe we can't deport 12 million illegals. But we don't have to make them citizens either.