I AM INDEBTED to the columnist George Will for a conceit of Paul Valery's -- that history is the science of what never happens twice.

James Barber would vigorously disagree with that. Barber is the kind of man who classifies everything. He sees individuals as types and events as part of recurring trends. In The Presidential Character he pinned the chief executives, like so many moths, onto four categorical frames: active or passive, positive or negative. In his new book, The Pulse of Politics, he finds an inflexible rhythm in the politics of presidential elections.

As he reads political history, an election involving a hot conflict, say the one between Truman and Dewey in 1948, is sure to be followed four years later by one concerned with "conscience." Four years after that, the voters yearn for "conciliation." Then back to conflict, and the beat goes on.

Thus, after Truman-Dewey came Eisenhower's "crusade" against communism, corruption and vacillation over Korea. A crusade, with likeable Ike in the lead, met the country's need for an expression of conscience. By 1956 there was racial turmoil at home and violent uncertainty abroad, in Hungary and at Suez. Americans looked for a "conciliator" in such a perturbed time, and found him, once again, in Eisenhower. We were ready for a scrap in 1960. Kennedy and Nixon provided it.

Politicians out of sync with this rhythm, as was the sophisticated Adlai Stevenson in a time calling for a crusade of conscience, do not succeed. Politicians who are able to transform themselves, or through their cunning manipulation of the media seem to be transformed into what the rhythm of the hour requires, can make it. So Nixon, the old gut-fighter, appeared in 1963 as conciliator to a nation torn by riots and in agony over the war in Vietnam. Through four elections FDR managed to be whatever was needed: a conciliator to a frightened nation in 1932, a warrior in the 1936 combat with conservative Alf Landon, the voice of conscience against Hitler in 1940 -- Wendell Wilkie was his ostensible opponent -- and a conciliator once again in the wartime election of '44.

It is an interesting design, though like most attempts to categorize history, it requires a certain suspension of disbelief. Was the Hoover-Smith election really fought on the ground of conscience? Was FDR really seen as a conciliator in 1932, or as simply preferable to his demonstrably inadequate opponent? If the 1976 contest was one of conscience -- clean Jimmy Carter versus Watergate -- will we in 1980 be looking for "conciliation" in a period of recession, inflation and foreign threats?

There do seem to be pendulums that swing through political, as through personal, history. There are conservative eras, and eras when we are amenable to or insist upon change. (Fewer of the latter, and shorter in duration: In early 1940 FDR told his friend Jim Rowe that he was dubious about his prospects because liberalism in America had an eight-year life span, and it had expired.) Sometimes the call is for broad participation in decision-making; sometimes we yearn for masterful leadership. Events interrupt the pendulum swings -- war, depression, civil disorder, official corruption.

And surely there are right and wrong times for most politicians. Winston Churchill was much the same figure throughout his long political life, but it required a coincidence of his belligerent personality and the ripened Nazi menace to make him prime minister. Edmund Muskie seemed the ideal Democratic candidate in 1970 when Cambodia and Kent State sent the nation's fever high, and a voice of cool reason was most welcome. But there was no presidential election that year, and by 1972 his party at the grass roots was eager for someone more extreme in opposition to all Nixon stood for. Needs and moods change; the public is more often affected in its choice of leaders by opposition to the alternatives than by allegiance to the winners.

So, to change the metaphor, the tides run, and above and through them break waves and opposing wavelets. Smart politicians discern those tides; smart and lucky ones ride with them to victory. James Barber sees the tidal flow as a 12-year passage through conflict, crusade and conciliation. He is right in arguing that there is a rhythm in public affairs. Contra Valery, enough happens twice to make a science of history worth pursuing. But Barber's formula is overly neat. He has tried to lay a grid on the protean sea, and as in the case of other theorists before him, the sea will not cooperate.

There is another major theme in his book, that the media have replaced the political parties in selecting nominees. Writers and publishers have exerted a strong influence on the public's perception of candidates since the founding of the Republic. Henry Luce and his Time-Life organization "created" Willkie, and helped make Eisenhower the inevitable Republican choice in 1952. Later candidates -- Nixon in 1968, Carter in 1976 -- played the media like a xylophone, the one packaging himself inside soothing spots on television, the other taking brilliant advantage, in the early primaries, of the press's fascination with the race itself, as distinct from the qualifications of those running it. Neither had much need for their parties. They went around them to the people, through the media.

When the press has intervened, it has often diverted our attention from the real stakes at issue by concentrating on the candidates' gaffes - Muskie's weeping rage in New Hampshire, Ford's boner about Poland, Carter's about "ethnic purity" -- because abusive conflict, not character and the issues, is meat and drink. When it should be focussing on the facts and holding the candidates responsible for their response to the facts, it has given us a race card -- did well in last outing on sloppy track -- and assorted gossip.

This is the indictment. What effect such press has on the rhythm of political history, on what the voters want at a given time, is not entirely clear. Barber seems to be contending that through a complaisant media, politicians may cause themselves falsely to appear consonant with the prevailing tide of public needs, and thus may fool us.

That we can be fooled is clear. That the press can be trivial, unfair and mistaken is also true. So can we all; but most of us do not visit those shortcomings on millions of our fellow citizens. Yet it is not the mischief that reporters can do that troubles me about our politics as they are affected by the media. Like most citizens, I regard what I read in the papers with a certain skepticism.

What troubles me much more is that television has converted politics into a spectator sport. Before the tube, we typically view politics, like everything else, on what Kierkegaard called the "esthetic" level -- the shallowest level of apprehension, unaffected by ethical or still deeper consieration. We like or dislike the show. We are nonparticipants in it.

Thus the muscles of our citizenship grow slack. We do not wish to do the necessary work of weighing our needs as a people against the character and policies of those who profess to meet them. If the contrived performance is good, we are pleased to tune in next time.

In the television age, it is unrealistic to long for a return to clubhouse politics and town meetings. But it is not too much to hope for a more searching journalism on the part of the networks; nor for a more serious commitment to the responsibilities of citizenship on the part of those affected by presidential politics. The latter would require thought about the complexity of the domestic and foreign troubles that beset us, and at lease some weighing of alternative strategies to meet them. It would require us, in this presidential season, to measure Carter's performance in choosing among those strategies, not against an ideally successful result, but against his rivals' proposals and rhetoric.

It is widely said that people feel alienated from their government. Small wonder; we are similarly alienated from the characters in sitcoms. But we are not impotent shareholders in the national corporation. A plurality of us determines the shape of our common future. It is serious work, and in the end the responsibility for it is ours alone.