SIX DAYS after his narrow victory, President-elect John F. Kennedy said: "It was TV more than anything else that turned the tide." Pollster Elmo Roper subsequently found that of four million voters who said their presidential decision had been based on the televised Kennedy-Nixon debates, almost three million voted for Mr. Kennedy. In view of the fact that his margin in the final national count was ony 112,000 votes, the televised debates were indeed crucial.

Richard Nixon obviously agreed with John Kennedy's analysis, for he avoided debates in his successful campaigns of 1968 and 1972. That has been the general practice of front-runners. If you're ahead, don't debate. If you're behind, challenge your opponent to debate, any place, any time. Ronald Reagan, following this counsel of caution, has refused to participate in tonight's Republican presidential candidate debate in Iowa, sponsored by the Des Moines Register and Tribune.

The Iowa debate may not be conclusive in the Republican contest, but it is important. All the major candidates -- minus Mr. Reagan -- will have a chance to be seen and heard in living color (thanks to PBS and CBS) by the voters of Iowa and the nation. The debate will let all of us judge the candidates in an adversary forum. Each will have a chance to answer every question. With no top dog carefully nursing every syllable for fear of a miscue, each candidate will regard tonight as a unique opportunity to emerge from the pack, to stage a breakthrough that will make him the principal challenger to the front-runner, Mr. Reagan.

We shall be following the debate with the keenest interest, remembering the words of a veteran of past debates: "If it hadn't been for the debates, I would've lost" -- Jimmy Carter, 1976.