I start with the postulate that the only legitimate function of government is to promote the welfare of its citizens. This is something at which they typically fail, but in my view it's the only thing they ought to be doing.
(image from photobucket.com)
If this is the case, then our immigration policy should be clear. We should only admit those immigrants likely to enhance the general prosperity of our economy. That would seem to consist of workers currently being admitted as temporary workers under H1 visas. If we can get them to stay, we should. They tend to be highly paid, and by definition they have skill sets for which there's constant demand, even in this economy. The argument has been made that this has the effect of depressing wages in the nursing and tech industries. By and large, I'm willing to allow that, as providing a more generalized benefit to the country than the cost it exacts from a subgroup of the nation's citizens. I think the health of the whole country suffers when there's a nursing shortage.
(image from eduinreview.com)
This logic does not apply to H2 visas, nor does it apply to illegal immigrants. The effect of increasing the labor supply in the fields of agriculture or construction or hospitality will again be to depress the average wage in those fields. But this increased productivity benefits a far more narrow segment of the citizenry than is the case with nurses or programmers. Meanwhile, the number of people whose wages are impacted is much larger in the case of low-skill, low wage jobs. Instead of the costs being born by a small segment of the middle class, the costs are born by the working and lower classes. And the difference between $8 and $12 per hour is MUCH greater than the difference between $20 and $30 per hour.
It is constantly claimed that "Americans don't want these jobs." That is demonstrably untrue. Americans don't want those jobs at that particular rate of pay. If you pay a busboy $12 an hour, you'll have a line of American applicants stretching into your dining room. If you can't afford to pay a living wage to your employees, you need to raise your prices. And, I want to see your books, because you're doing something wrong.
On the other hand...
it's awfully hard for me to justify telling Mexicans that they can't look for work in parts of our country that we stole from them, in an unjustifiable war of aggression aimed at creating more slave states.