The raw excitements of summer are yielding to the yet rawer ones of autumn as the presidential campaign careens lickety-split and right on schedule into a familiar cul-de-sac: a debate of sorts, about debates of sorts. Is this trip necessary? It is inevitable. It is not beneficial.

There will be only two too many debates this year. That is, there will be only two, as George Bush insisted, rather than the four that Michael Dukakis hankered for.Bush, following the rule that you should hang a lantern on your problem rather than try to hide it, has been happy to play the hapless fellow.

His campaign has speculated it would be best for him if the debates occurred during the Olympics and World Series so they would not have the nation's undivided attention. And the last one should occur well before election day so Bush will have time to recover from any rhetorical pratfall, any self-inflicted Pearl Harbor.

This Republic prospered from the presidency of Washington through that of Eisenhower without presidential debates, and there was, mercifully, a 16-year hiatus between the first set (1960) and the second. So why, aside from the fact that television exists, do we have debates?

In a fascinating volume of essays, "Presidential Debates: 1988 and Beyond," the editor, Joel L. Swerdlow, says that neither in democratic theory nor in practice were debates between candidates considered important, until recently.

He notes that debates were defined as aspects of legislative deliberations, not as instruments of mass persuasion or electoral campaigning. The Lincoln-Douglas debates in the 1858 senatorial campaign were the first in of national significance and they did not inaugurate an era of debates.

It is said in defense of debates that because they last longer than 30 seconds they at least counter the trend toward trivializing compression in political discourse in this age of "sound bites." The trend is deplorable. But debates, being incoherently episodic, are part of the problem.

Kathleen Hall Jamieson, a contributor to the Swerdlow volume, notes that voters have voted with their television dials, emphatically rejecting long - nowadays more than 60 seconds - political events. On Sept. 22, 1964, they gave a landslide victory to "Peyton Place" and "Petticoat Junction" over a conversation between Eisenhower and Goldwater.

On Nov. 4, 1979, they chose "Jaws" over Roger Mudd's interview with Ted Kennedy. In 1960, viewership declined sharply for the fourth and final Kennedy-Nixon debate.

Broadcasting, says Jamieson, has brought a steady decrease in the length of political messages. "The 30-minute speeches that reached the nation's radios were standard fare by 1952, the year of America's first telecast presidential campaign.

By 1956, the five-minute speech and spot were edging out the longer speech. In 1964, the five-minute speech gave way to the 60-second spot and then the 30-second spot." In a wired nation, the political imperative is "survival of the briefest."

Debates reflect this. They are tossed salads of brevity.

Because neither side is ever sufficiently confident to relish the thrust and parry of real debates, our debates are, inevitably, parallel press conferences. They test skills unrelated to the real tasks of governance. (Presidential press conferences, wisely de-emphasized by Reagan, are similarly overrated as useful events.)

Debates are supposed to test a candidate's ability to "think on his feet." But debates are primarily the regurgitation of market-tested paragraphs. Reflexes, not thinking, are crucial. Anyway, who wants a president thinking on his feet? The presidency is not a solo act. Successful presidents surround themselves with specialized talents and act in concert with them.

This year's debates will be bracketed by sound and fury - the hype before and the spin-doctors after - and will signify the institutionalization of yet another idea that diminishes our politics.