The best defense of Arizona’s right to enforce federal immigration law that I’ve seen thus far comes from the Tenth Amendment Center, whose article by Bob Greenslade says this:
The battle raging between the federal government and the State of Arizona over its so-called anti immigration law has raised some constitutional issues that will ultimately be decided by the United States Supreme Court. It has been asserted by the federal government that the States are precluded from protecting their borders and controlling illegal aliens because the Constitution grants the federal government these powers. This assertion is erroneous because the individual States, as sovereign political entities, have the absolute right to protect their borders from illegal aliens irrespective of the Constitution or any power granted to the federal government.
The article continues, quoting the U.S. Constitution, court decisions and legislation to make its case. A worthwhile read, which you can find HERE.