Working for the (Expensive) Clampdown
Ryan Avent points out that Prince William County is finding that big crackdowns on illegal immigration cost a lot of money, remarking "Prince William would be far better off identifying the people who claim to be harmed by immigration and figuring out how much money it would take to shut them up."
A joke of course, but it illustrate the general shape of things. The evidence suggests that illegal immigration is damaging to the economic prospects of the least-skilled Americans. It also suggests, however, economic benefits to the majority of Americans who don't fall into the "least skilled" category. On top of that, it has large benefits to the immigrants themselves. Crackdowns, meanwhile, have direct financial costs whereas putting illegals on a path to citizenship has direct financial benefits (fines and taxes). Under the circumstances, crackdowns on illegals is an incredibly inefficient way of helping the unskilled. Most crudely, you could just cut them checks and make everything pareto-optimal. More realistically, improved social services, etc.