Lively. Enlightening. And helpful to voters as they settle in for a long 1984 . Sunday's three-hour televised debate among the eight principal Democratic presidential contenders was all of that. It was also the first major Democratic political event of this election year.
For some time this newspaper has called for debates that burrowed beneath pat campaign speeches and delved into immediate issues in the political race. This get-together accomplished that, probably in large part because of its length. More such debates-in-depth during the political year would greatly benefit American voters.
Yet the length had a drawback: How many Americans sat glued to their PBS stations all three hours?
Those who did saw differences emerge among the eight - on arms issues, jobs, and economic questions.
Direct interplay between candidates revealed, too, distinct differences in approach: who was aggressive, who presidential. Who was defensive, projected basic decency, or adroitly used humor.
As the front-runner, Walter Mondale entered the discussion with the most to lose. A new poll showed him with a 49 percent to 20 percent lead over John Glenn in the Feb. 20 Iowa caucus; he is also considered the leader in the Feb. 28 New Hampshire primary. Several of his platform rivals were unknown to almost all Americans; if the former vice-president were to stumble, they stood to benefit from this nationwide TV exposure.
He did not particularly stumble Sunday, although some viewers may not have been pleased when he took up Mr. Glenn's aggressive challenge and hotly denied that the Mondale approach was to overpromise. Yet to have let the charges go unchallenged might have been more damaging in the long run. Some might say Glenn was doing the Republicans' 1984 work for them, in playing to Mr. Mondale's courting of labor, educators, and other Democratic power blocs. But it's an issue Mondale will hear often, and the price of his path to front-runnership.
The sharpness of the Glenn attack, however, which came in the last third of the program, may have made more impression with voters than the content of his charges. If they judged his approach to have been mean-spirited, the man often thought to be in second place will prove to have been harmed by the performance.
Gary Hart, not yet a factor in the race, made a more persistent attack on the leader. He runs an even more serious risk of having the style of his attack held against him.
In a very real sense the biggest winner was the voter who watched, and made his or her own judgments.
The initial impression is that, among all the candidates, the Rev. Jesse Jackson appeared to gain the most, showing a grasp of a wide range of issues. He settled in the affirmative the question of whether he belongs on the same stage with the seven others.
Other candidates made themselves and their views known. All have now emerged from the category of the unknown, among voters who watched the program. In that sense each benefited from the program: The challenge now is to move from recognition to voter strength.
Beyond this, it would be a mistake to calculate overmuch the short-range pluses and minuses for each candidate. It will take a while for reactions to the candidates' performances to sift down. Then, too, the real importance of the debate is not who ''won'' or ''lost,'' but the exhibiting of the entire spectrum of Democratic alternatives to President Reagan, including something of the programs and personality of each candidate.
Finally, much of the balloting in presidential caucuses and primaries is done by party activists, who represent only a portion of Democrats.
At this time most voter support for Mr. Mondale and other candidates is largely ''soft,'' and could switch to another candidate. Most primary voters have not made irrevocable decisions, if any decisions at all.