Quayle was simply incapable of going beyond his script. Which means one of two things. Either he is too shallow to have original thoughts--in which case he is intellectually unfit for high office. Or he has thoughts, but is too terrified to risk expressing them under the pressures of the presidential arena--in which case he is emotionally unfit for high office.
In either case, the Quayle factor endures. Bush is left with only one consolation: People know that if enough of them vote Republican they might some day get the insubstantial President Quayle, but they also know that if enough of them vote Democratic they are sure to get the insufferable President Dukakis.