In his monologue last Thursday night, Johnny Carson was talking about the League of Woman Voters:

"They say that if Carter doesn't show up for that debate, they're going to put on an empty chair, while Reagan and Anderson debate. You know what bothers me? Suppose the chair wins." The studio audience in Beautiful Downtown Burbank broke into a big laugh.

In the television business, Carson's monologue is regarded as a bellwether -- "the best news analysis on TV," as one network executive put it this week. In this instance, Carson did appear to be a weathervane; his joke pointed in the direction of a prevailing wind that began blowing through the mass media almost immediately afterward.

At first blush, President Carter's decision -- announced a week ago last Tuesday, two days before Carson's joke -- played horrendously for the president on television and in the papers. The morning line was that Carter had blown a big one -- run away from a fight, staged a juvenile temper tantrum, shot himself in the foot.

Carson's joke suggested otherwise, and lo, within days the whole tenor of the mass media did, too. As of today, at least, the significance of Carter's decision not to debate seems to have diminished considerably. The media have failed to sustain the issue as an important matter.

Now the League of Women Voters has decided that an empty chair would unfairly disadvantage Carter, so there will be no empty chair.

ABC has apparently decided that it would rather show a big Hollywood movie -- "Midnight Express" -- than a debate between Republician nominee Ronald Reagan and independend John B. Anderson, so the nation's television viewers will probably have an attractive choice this Sunday night, one that could diminish the audience for the debate by tens of millions.

Risks remain for Carter -- ABC can still change its mind, the content of the debate could still damage the president, there might still be a significant popular backlash against the president for his refusal to debate. But today it looks as though Carter may be winning the biggest gamble of his still-young campaign.

The Carter camp expressed satisfaction that the cost so far seems low compared to the risks of bolstering the Anderson candidacy. For reasons underlined by many recent polls, Carter is convinced that his first priority must be to weaken Anderson, and that a three-way debate could only have the opposite effect.

It was always a long-term gamble. The Carter camp said from the beginning that it would suffer some short-term negative consequences. But the first available polling data suggest that the damage is not yet substantial. In a new poll showing Carter pulling ahead of Reagan for the first time nationwide, CBS and The New York Times found that one in six undecided voters felt that Carter's decision not to debate would make them personally less likely to vote for the president on Nov. 4. The poll was taken last week between Wednesday and Saturday and election day is more than six weeks off.

Following the course of this story in the mass media shows how some events can lose their appeal as news because of the weight of their initial impact. In the effect, the original jolt tends to send the story to the inside pages or off the network's evening shows within a few days.

For three days last week the debate and Carter's refusal to take part were the big story on the evening news programs and in the papers. This ended Thursday. Once the news was conveyed and initially analyzed, it seemed, there was nothing more to say. In the days since, the debate story has been mentioned in passing or not at all. If Ronald Reagan hadn't continued to talk about it, the issue would have disappeared completely for several days.

Last Friday night only ABC's evening news even mentioned the debate issue -- in passing, deep in a report on Anderson. The other two networks ignored the matter entirely. The Sunday Washington Post carried no news story mentioning the debate issue; The New York Times carried one.

An issue that can sink so far so fast is not easily revived. Of course, the debate will return to the news shows and the front pages as it approaches this weekend. But the crucial fact would appear to be what didn't happen: This was a case of the furor that wasn't.

For one viewer who has everything on the network news this month, the most interesting moment came Monday evening of this week. The NBC Nightly News devoted its "special segment," the longest feature on the show, to a historical look at debates in politics, starting with the original model, Lincoln vs. Douglas in 1858.

It was a lively piece of television, and it conveyed one clear message: In all American history, only Lincoln and Sen. Stephen A. Douglas really debated each other -- the modern version isn't really a debate. With a panel of outsiders asking the questions, said correspondent Fred Briggs, it isn't the genuine article. When more than two candidates join in, "is it a debate or a forum?"

Journalists themselves often have trouble explaining how a story took the shape it did -- why the NBC special segment came out this way is anyone's guess.But it is easy enough to imagine a very different special segment -- one beginning with the observation that direct encounters of presidential candidates have become enormously popular in America, that Carter got elected because of them and long promised to debate his 1980 opponent(s), but that now the prospects for debates (or forums, or whatever) in 1980 are diminishing.

In other words, NBC might have done a piece saying these encounters were important and useful, and that Carter was jeopardizing their survival. Instead, the network implied that Carter's decision was -- if not innocent -- far from egregious.

Both the networks and the newspapers were easily distracted from the debate story after those first few days last week. There was othe political news that go more attention -- the new from Iran about the fate of the hostages; the charge that Carter's campaign manager, Tim Kraft, has used cocaine; Carter's harsh attacks on Reagan; reports that Reagan's campaign was operating more smoothly; and new polls showing the presidential race close.

The impression should not be left that Carter got clean away with his decision to duck the debate. One segment of the media gave it to him 'twixt his blue eyes day after day -- the nation's editorial pages. Cartoonists and editorialists had a heyday, denouncing Carter for immaturity or cowardice or a betrayal of the American way.

But reading these denunciations produced the feeling that they were all very predictable, and unlikely to make much difference. This year the editorial pages tend to be more anti-Reagan than anti-Carter anyhow, and the debate issue won't change that.

Perhaps the last word on this matter cam from an editorial page piece written by a nonjournalist in this newspaper lst Sunday. The author was John Sears, once Reagan's compaign manager.

Carter's judgement in refusing to participate in three-man debates is sound," wrote Sears, ever the calculator.

"But whether it turns out to be a mistake or not will depend on how he defends his position and whether he can come forward with a heavy dose of positive news during the next week or so. By making news himself, he can make the Reagan-Anderson debate look like a contest of also-rans . . . The important thing is not whether you debate but rather the perceptions that you allow to grow out of the circumstances."