Is there any basis in the Constitution for denying rights to a particular class of free citizens? It seems that with regard to this argument there should be two classes of individuals: those who pose a greater-than-average threat of committing violence, as judged by past behavior; and those who don't. The first group can be further subdivided into the sane and the criminally insane, but in either case they belong off the streets. As for everyone else, the Constitution should apply as written.
It is wrong to assume that anyone labeled with the broad brush of "mentally defective" is going to go berserk and start shooting people. Are we going to deny someone his rights on the basis of low self-esteem, bulimia, or a learning disability? On the positive side, this law would mean no armed body guard for Rosie O'Donnell!
One more thought: In the past, homosexuality was viewed as a mental defect. I wonder how a “no guns for gays” law would sit with liberals ;-)