The pre-packaged president

Does television actually sell presidents as it sells toothpaste? That's the question Bill Moyers poses in The 30-Second President (PBS, Wednesday, Aug. 8, 9-10 p.m., check local listings), still another excellent program in the landmark series ''A Walk Through the 20th Century.''

Moyers's conclusion is ambivalent -while TV can certainly trivialize the issues, the fact is ''it's here to stay'' and it should be used ''in a way that does justice to the ideas and serious purpose of the men and women who run.''

But is it used that way?

Moyers can't seem to come up with a single answer, despite the fact that he interviews two of the leading experts on the use of commercials in political advertising: Rosser Reeves, who pioneered the field with Gen. Dwight D. Eisenhower's campaign; and Tony Schwartz, who has worked on the campaigns of most of the Democratic nominees since General Eisenhower's day.

Says Mr. Reeves: ''It was inevitable that it had to happen. I was merely the first one that did it.''

Says Mr. Schwartz: ''All the important things in our life have been restructured by television. We find that the political parties are no longer the major communications force in politics; the networks are. You might say the three parties are ABC, CBS, and NBC.''

When Moyers protests that the purpose of a political spot should be to let you know what a candidate will do about a problem rather than how he feels about a problem, Schwartz responds:

''No one has had experience with the answers to our problems. People tend to feel that if a man is equipped for the job and feels as deeply as they do about something ... he'll do the right thing.''

''A Walk'' is a continuing joy to watch, despite its unsettling character and even when it occasionally results in inconclusivesness, as in this one. It still deals with fascinating material, handled with sincerity and compassion by a man who listens intently to what his guests are saying - even if, as is the case occasionally on this show, what they are saying is nonsense.

Bill Moyers, who served as President Lyndon B. Johnson's press secretary, is honest enough on this show to confess a great reservation about that period: Although it was apparent that Vietnam was the major issue of the day, ''Vietnam was never touched in any political spot in 1964,'' he observes sadly.

Moyers says that quietly. But it makes more of a point about the value of political commercials than anything else said on this disturbing program.