IF JOHN KERRY hadn't already clinched the Democratic presidential nomination, his medals meltdown on "Good Morning America" this week would have sunk his campaign. Much as Howard Dean's crazed "I Have A Scream" speech jolted voters into wondering whether someone so hotheaded should be allowed anywhere near the nuclear trigger, Kerry's abusive tirade on ABC gave millions of viewers a foretaste of how far presidential discourse will sink if Kerry becomes president.