Well, the idea being that President Bush wants to win fair and square, and really doesn't need this particular character issue in order to win; a gentleman should be able to win against a scoundrel without having to point out what a scoundrel his opponent is. While refusing to engage in these attack politics and the coarsening of public discourse, the President is at the same time forcing Democrats themselves and everyone live up to higher ideals.
After all, his first concern is always for the integrity of America and her people, and only lastly for himself.