Here's a Washington horror story: Two Mo Udall staffers were in a restaurant during the congressman's 1976 presidential campaign, and as they were sitting there, eating and drinking and just dumping all over competitor Scoop Jackson, two women came in and sat down. Two inches away, probably.

"Anyway," recounts one of the staffers, "we kept on talking about Jackson, saying dumb things like 'God, is he a fascist pig' and so forth. Really awful stuff. We went on and on and on. Then a friend of mine came in and looked at one of the women next to us and said to everybody:

"Oh, do you two know Helen Jackson -- Scoop Jackson's wife?"

Washington can be a dangerous town, and not just because the Russians have targeted the Pentagon. The real peril here -- or irresistible delight, depending on whether you're talking or listening -- is eavesdropping. In restaurants, in bars, on airplanes, in bathrooms, just about everywhere as the fall social season begins in the capital. When informaion is power, no place is sacred.

"Nobody who has got his right mind about him talks about anything sensitive in an elevator or restaurant in this town," says J. D. Williams, a well-connected lawyer and lobbyist. "There is an active network in Washington that trades information on what is heard over lunches and dinners."

"I just got back from Maison Blanche," Tommy Boggs, another lawyer and lobbyist, was saying the other day, "where I was trying to have an intelligent conversation with a high-ranking official in the State Department. At the next table were Lesley Stahl and George Herman from CBS with Jack Watson from the White House. Behind me was Mel Elfin of Newsweek. I quit talking."

Not everyone does. Larry King, who wrote "The Best Little Whore-house in Texas," once worked on the Hill and definitely should have known better. But he can readily recall his own tale of horror:

"I was drinking or drunk or both at the Class Reunion one night," he begins, "and I was babbling on -- this was when I was still going with my present wife, who was working for John Connally's Washington law firm, and Connally, you know, just happens to be my arch enemy -- and I was just spouting off to a bunch of my friends about how I was going to marry Barbara so I could be Connally's partner-in-law. Well, there was some reporter sitting next to us and the next month my entire conversation, profanity and all, showed up on the gossip page."

Reporters are a relentless fact of life in Washington, which probably has a higher ratio of media people per square foot than any city on earth. These folks grow up as instincitive eavesdroppers, blossom into professional eavesdroppers and then, upon finding themselves placed next to something as delicious as a hot government source eating manicotti and talking relentlessly, become uncontrollable eavesdroppers. eTheir eyes glaze over in bliss. They'll write on the napkin.

Not that government sources are so innocent. They'll eavesdrop just as gleefully on reporters and anyone else, simply because it's important to know who's got what on whom. "You'll be out, and you'll hear somebody say, 'Oh, did you see the new poll?'" says a certain White House staffer. "So you put your ear up and you tell the person you're with to shut up."

Included among Washington eavesdroppers are lawyers, lobbyists, advertising people, doctors, an occasional artist, maybe even a bass player who's to perform at a Capitol Hill fund-raiser -- anybody, really, whose life touches the heart of a small company town.

It's a town where former State Department spokesman Hodding Carter III says he learned to talk, eat and listen at the same time, and where Transportation Secretary Neil Goldschmidt insists that "having been told the way things are, I eat in a closet." FBI Director William Webster does eat out, but never talks shop; Hamilton Jordon, who learned the hard way, acts dull or stays at home.

"When I go out to social events, I'm pretty boring," he explains.

Still, when eavesdropping in Washington is as vital as it is voyeuristic -- and when everybody knows somebody who knows somebody else -- leaks occur. In fact, dams burst. "Uncle Sam Unlimited," says Liz Carpenter, an assistant secretary of education who knows everybody who knows somebody. "It's behind-the-palms intelligence."

Or behind the airplane seat. A politician tells this story which absolutely does not have a happy ending: "A congressman on a certain committee was on a nonstop flight from California to Washington, and really running down the committee's chairman. Just calling him all sorts of things. Well, sitting in front of him on the plane was the chairman's oldest friend. When they hit the ground, he was on the phone in about five minutes, telling the chairman everything. Nearly wrecked the guy's career."

But bear in mind, not all eavesdropping is as tragic. Or dangerous. In fact, some eavesdropping (on the part of the eavesdropper, not eavesdroppee) can range from merely interesting to downright fascinating.

"I was having lunch one day at the Hay-Adams," says a former intelligence official who once made a living eavesdropping, "and across the restaurant from me was John Anderson with Mary Crisp [who was soon to become Anderson's national campaign chairman]. This was the day he went to meet with Ted Kennedy. I was reading his lips while he was talking and I gathered from what he was saying to her -- you know, you only get about 70 percent of it, intermittently -- that he was going to find out how enthusiastic Kennedy was going to be in his support of Carter.

"Then I went back to my office," continues the delighted eavesdropper, "and I said, 'I'll bet I get a call from one of my aides on the Hill saying John Anderson's going to meet with Kennedy.'" And he did.

Often, the best reason for eavesdropping is that the conversation at the next table is a good deal more interesting than the one at yours. A Washington writer was at New York's Russian Tea Room with an offspring recently, and suddenly heard this wafting toward his salmon:

"I told my daughter I'd give her $10,000 if she wouldn't marry the guy."

"And did she take it?"

"Well of course she did!"

Says the writer: "We got so engrossed in this conversation, we couldn't hold our own. So we just sat there eating -- and listening."

Then, too, eavesdropping sometimes turns up jokes you can steal and add to your own repertiore. "I was at the Capitol Hill pool one day," says Corcoran Museum Director Peter Marzio, "and Afred Kahn was there, flailing around. I mean, he can barely swim, right?

"Well, this guy standing there says to another, 'You know who that is? That's the president's economics expert. And you know, he swims better than he can fight inflation.'"

Finally, there are those who not only eavesdrop, but eavesdrop on the record. In other words, they quietly monitor a particular conversation and then, as the mood strikes them, burst right in.

For example, Nina Straight, who is an Auchincloss, a half-sister to novelist Gore Vidal, a law school student and a person who laughs a lot, once was laughing a lot at Le Steak in Georgetown. This was because she and her son were doing their own imitations of Roseanne Roseannadanna from "Saturday Night Live." Lo and behold, the nearby table of four joined in.

"Suddenly," she recalls, "we were having a dinner party for six -- when there were just two. It's sort of amazing. Or familiar, I think is the phrase. A tad familiar."

And just a few weeks ago this summer, two Washington friends were having dinner with a New York public relations consultant at the Laundry in East Hampton. The conversation had taken a serious turn to the Holocaust. Suuddenly, a woman sitting at the next table turned to them and screamed:

"SHUT UP!"

Generally, the best place to eavesdrop in Washington is restaurants. The tables are close together, and, if you select a trendy spot, chances are good you'll find yourself sitting next to a trendy celebrity. But be sure to know your faces.

Not long ago, says a Joe and Mo's waiter, a woman having lunch there overheard an interview between a newspaper reporter and somebody on television she couldn't quite place. "I know you," she finally said to the somebody on television. "I know you."

"I'm Dan Rather," replied ABC's Sam Donaldson.

Bathrooms are also an excellent place to eavesdrop. Rep. Charles Wilson (D-Tex) swears by these, particularly Capitol Hill bathrooms. "You sit in the stalls and listen to what the guys at the urinals say," he explains. "I've heard a lot of the Republicans bemoaning Reagan's China thing."

The New York-Washington Eastern Airlines flight is fine territory, too.

"The shuttle is a place you can always eavesdrop," says a White House staffer. "You've got reporters on it, politicians, business people . . . "

Then, too, there was the floor of the Democratic convention at Madison Square Garden. Very ripe.

"I would just walk around on the floor and listen to things," says a Carter campaigner, "and I'd hear something interesting, and pretend I was standing there watching something else, my head back, for about 10 minutes. Then I'd run to the boiler room and I'd say, 'Hey, I think there's something there we should check out.'"

Which is just one method of many in the eavesdropping business. Other techniques, culled from beginners, novices and professionals, include:

Listening and talking at the same time, which is Hodding Carter's technique. "People who know me know that I can be listening to what's going on at the next table, even when I'm doing the talking. A certain look comes over my face."

At art openings, peer closely and intently at a centrally hung painting. You can do this undisturbed for hours, and at the same time, hear everything that goes on.

Invest in supplementary eavesdropping devices. Jerry Kosinski, the author of "Being There" and a close observer of the Washington scene, eavesdrops regularly for material for his novels. He says he has purchased a "Big Ear" listening device that he found in a children's toy catalogue, listed as a parabolic microphone. It picks up and amplifies sound from 200 feet away. He says he's attached it to his New York balcony.

"To a novelist," he says, "any whisper you can pick up is worth it."

Learn to read lips. Kosinski does this too. "You have to," he explains. "You must remember, this is the country of Muzak."

A lip-reading hint from the intelligence official, who explains that it's not necessary to go to spy school to learn his art: "Just turn on the television and practice by seeing if you can follow what somebody says."

Don't talk shop at lunch. David Margolis, chief of the organized crime section at the Justice Department, has learned to discuss other topics."Sports and Women," he says. "Or, the reverse order."

Better yet, don't talk shop, period. FBI Director William Webster, an immigrant from St. Louis, learned this quickly. "When I first came to Washington," he says, "I didn't realize how many people go to parties and don't ask,'Oh, aren't the flowers pretty?'" Back in St. Louis, he recalls, society reporters were interested in good floral arrangments -- not good dirt. Now, he says,"this is just not a good place to talk about investigative matters."

Learn to speak a foreign language. But not french, not Spanish, maybe not even Finnish, because there are lots of embassies here. What you need is a real foreign language, like Serbo-Croatian. But then you have to teach it to your lunch date.

Remain paranoid. It keeps your guard up, and besides, it's an easy state of mind in this town.

"Hey, there's no special reason I happened to be called about this story, is there?" says Tom Quinn, a lawyer who raised money for Ted Kennedy but is now doing it for Carter. "What have I said? Do you know something I don't know?"