First - this is amature hour at the White House yet again. Having Hillary let the cat out of the bag in South America on June 8th and no one from the WH directly contacting Arizona's governor? Reminds me of high school.
Second - I don't think the actual law itself is un-Constitutional. I will bet that the DoJ will argue that the State doesn't have the right to pre-empt the Federal Government by issuing the law, as border security is under the jurisdiction of the Fed and not the State. On those grounds, the DoJ may have a case against the passage of the law.