[Prokopton]

"The cost of rounding up 12-30 million illegals is ignored, but enforcing the law is a 'feel-good' kind of idea that everybody can get behind."

So enforcing the law is just a "feel good kind of idea"?
What country are you from? I'm a United States Citizen which is a Nation of Laws. Enforcing those laws is the duty of government. If the laws are not enforced by the government officials whose duty is to enforce them, these officials are shirking their duty and should be replaced with people who will. This includes the President and Congress.

[RightWhale]

There is a cost of doing this, and it is in addition to the cost of running ordinary programs. If somebody thinks the existing staffing level of enforcement is sufficient, they are encouraged to take a look at their own municipality and propose a sweep using the handful of agents currently available.