IF THE POPES could give up their sedia gestatoria, the sedan chairs by which they used to be carried into St. Peter's Basilica, Americans can give up political conventions. Anachronism when practiced by statespeople lends itself too easily to ridicule.

If a generation is about 30 years, then it has been nearly a generation since a convention of a major American political party conducted any other business save ratifying a foregone conclusion. The year 1952 was the last time it took more than one perfunctory ballot to name a Democratic presidential nominee.

Thirty-two years have elapsed since the Republicans needed more than one ballot to pick their boy. Almost no one gathered at Detroit, Ronald Reagan excepted, will be old enough to have attended a Republican national convention where the outcome was in doubt.

The national political convention has fallen into disuse as a decision-making body and has become much more an expensive, ho-hum reunion for journalists, whose numbers will obliterate the convention delegates. We all know what has brought about this evolutionary change in our political process. We denounce it often enough, but because we don't care to recognize its consequence, we cling to these quadrennial bashes. The change, as though you had to be told, is the primacy of the primary.

It's been a long time coming. Born of the pre-World War I Progressive upsurge, the presidential preference primary saw its first extensive use in 1912. That year, incumbent President William Howard Taft was butchered by ex-President Theodore Roosevelt, but the notion that the winner of the most primaries should axiomatically be handed the nomination was foreign to the settled political orthodoxy of the time; Taft was nominated. More states adopted the primaries in the next few years, but then in the '30s and '40s, the movement began to shrink.

By 1952 primaries seemed a thing of the past. That year Democratic Sen. Estes Kefauver won almost every primary he could enter, and they almost refused to give him a ticket to the convention. But Kefauver was to be the last decisive primary winner whose party would dare deny him the nomination.

In the years between then and now, more and more states would institute a primary, and gradually a new Washington political apothegm would be coined: You can't win the nomination unless you enter the primaries. In due course, that has been changed to: You can't win the nomination if you don't win the primaries.

This year there were primaries in 35 states in which 32.3 million people voted and nearly $150 million was spent (about 20 percent of which was federal money). The convention is out of sync in this age of popular sovereignty, when the people -- bless their woolly little heads -- choose all and everything.

Either nominees are picked by primary or by convention. It can't be done both ways. And since conventions stink of old-fashioned, Pullman car words like "caucus" and "smoke-filled room," "bosses," "steamrollers" and "political machines," the choice is for purity and the primary. That's what Senator Teddy doesn't grasp. Millions of people were beckoned into voting in the primaries on the premise that they, not a tacky politician, would choose the nominee.

H. L. Mencken considered the national political convention the most glorious and gloriously entertaining circuis in American popular culture. And it was. Nor was it the immorally rigged and manipulated meeting it is now thought to have been. But what made it our most beloved and exciting spectacle was that it was Barnum & Bailey with a purpose. It has none any longer.

Even those platform "fights" are a put-up job. The already selected nominee invariably controls the platform committee. The impotent dissidents are given just as much as is necessary to achieve party unity and a satisfactory public profile.

The lack of serious business to transact is reflected in the decline in the importance of the delegates. Years ago, senators, representatives, governors and other personages crowded convention aisles along with the anonymous goofball delegates with funny hats and hoarse voices. Now they only come if they have a pathological need to appear on national TV or they are close associates of the candidate's, important campaign operatives such as a man like Sen. Paul Laxalt is to Gov. Reagan. The modern political convention stands in the same relationship to the already chosen nominee as the Roman Senate did to the Emperor Augustus. The delegates don't come to deliberate but recreate, they don't come to designate a candidate but to celebrate him.

A lot of fine things are useless. Why not preserve this public entertainment even if it has degenerated into little more than a political version of "Aida," an opera that does have live elephants, although no donkeys on stage? Once purposeful public pageants that have become expansive and expensive ritual are justified as theater, and that's the objection. This is lousy theater. You can't put a drama on the air for five full days of prime time that doesn't have any suspense or any plot. We know the ending even before Dr. Cronkite blinks the CBS eye to welcome us to hours -- nay, days -- of ennui.

For this, millions and millions are going to be spent. You could even get righteous about throwing all that dough away in the center of downtown, poverty-stricken, unemployed Detroit. Let every Republican stay home from Detroit, buy an American car and put an automobile worker back to work.

But as the old political maxim says, if you can't give 'em bread, give 'em circuses. Only the clowns should be funny. The people you will be seeing on your TV screens at the conventions may have red noses, but funny they're not. And certainly never eloquent. The last speech given at the rostrum of one of these conventions which had any serious claim to oratorical quality was delivered in 1960 by then-Sen. Eugene McCarthy on behalf of the third, final and that time failing candidacy of Adlai Stevenson.

The modern way of dismissing something is to say, "It's only a media event." In this case, the media are the event. Not even professional politicians can name more than a dozen or so delegates to this Nembutal bash. Not one American in 10,000 -- this writer included -- can tell you the name of either keynote speaker or convention chairman or is stung with the curiosity to find out.

The TV screens will show the arrivals of the drudgy suckers who at their own expense have come to be bodies filling up seats on the convention floor. Absolute nobodies who will be rhapsodized as sincere, concerned, thoughtful citizen-politicians. All of which they doubtless are, but thoughtful and concerned doesn't mean intelligent, or they'd know they're supernumeraries in this opera.

The big players' arrival you don't see. When people like Barbara Walters or John Chancellor come to the amphitheater they zoom up in chauffeured limousines past the delegate drudge-trudges. It is fitting, for the solo tenors and the coloratura sopranos here all work for the media, maintaining your right to know even if you'd prefer to kill yourself to get out of knowing.

The number of reporters, editors, copy readers, videofont operators, media technical personnel -- not to mention friends of the children of the network presidents skipping about with press passes on -- far and away exceeds the delegates. The actual ratio this year is nearly 4 to 1 for the Republican convention (an estimated 12,000 to 15,000 with media credentials v. 3,988 delegates and alternates) and 2 to 1 for the Democrats (9,000 to 11,000 journalists, 5,436 delegates and alternates). Pity the poor delegates, who will be chased down hotel corridors and, if they're lucky, avoid being lynched or accidentally fired by NBC.

According to Liebling's Law (formulated by the late A. J. Liebling, professor of astro-journalism at The New Yorker magazine), a news event is inversely important to the number of reporters assigned to cover it. This explains why 10-candlepower, black-and-white TV stations from Michigan's Upper Peninsula send a team to cover or smother this archaic political rite.

Pending its abandonment, treat it as journalism's old home week where people in the business renew acquaintances and, having exhausted the local angle, the issue angle, the candidate's wife's angle, and the angle angle, come to accept that where there is a void, there is nothingness.