Forty years ago--the last time Democrats met in Los Angeles to nominate a candidate for president--there was only a small possibility that front-runner John F. Kennedy would leave without his party's nomination. This time there is not the slightest chance of uncertainty, let alone upset, in the official proceedings. The script has been written, the players rehearsed, the stage set. This week, while enjoying the din of predictable acclaim within the convention hall, Al Gore would be well advised to keep one ear cocked to the hum of unpredictable discord without. As Kennedy would learn in 1960, there are times when even the most carefully rehearsed presidential candidate could do worse that throw away the script. Maurice Isserman, a Professor of History at Hamilton College in New York, Is the Author of "The Other American: the Life of Michael Harrington."