BUSH DEFENDS AD THAT ASSAILS GORE
Gov George W Bush defends Republican advertisement that challenges credibility and character of Vice Pres Al Gore; says his pledge to change tone of politics will not prevent him from raising questions about Gore's truthfulness and leadership abilities; says commercial, which brings up Gore's 1996 fund-raising appearance at Buddhist temple, is way to undermine public confidence in Gore's own accusations against him; Bush photo (M)