Such a law would violate the 10th amendment.
Remember though, every law is constitutional until the court says otherwise. The biggest danger, as I see it, to an Obama Presidency would be who he'd have a chance to put on the Supreme Court. The good news is that even in a Democrat controlled congress, I think he'd have a hard time getting something like that through. There are likely too many moderate and conservative Democrats that remember what the '94 AWB did for their control in congress to pass such a measure.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar