IN LOS ANGELES, where people don't even watch the news, the latest thing in T-shirts asks simply and plaintively from chests-about-town: "Oh God -- what are we going to do without Walter Cronkite?"

On March 6, Cronkite will say ". . . and that's the way it is" for the last time as an anchorman of "The CBS Evening News," and America will lose not only its most trusted newscaster but a virtual human society blanket.

At ABC and NBC, however, no one is dreading the day that Cronkite turns over the reins to hair apparent Dan Rather. Both networks think this calamitous occurrence will give them an opportunity to snare new audiences for their own evening newscasts -- which have trailed Cronkite in the ratings for more than a dozen years.

In fact, you can't expect ABC News and NBC News to make more of a fuss over Cronkite's departure than CBS News does. CBS is now launching an intensive ad campaign to downplay the fact that Uncle Walter is ankling the anchor chair. After all, they don't want viewers to think an era is ending (though it is) and go wandering off to sample the competition.

So the CBS News ad campaign will stress the continuity of CBS News, trot out the proud traditions ("traditions that are no longer there," says one former CBS News employee) and refer to Cronkite as "our newest correspondent." Although he is relinquishing the anchor throne, Cronkite will still appear occasionally on CBS, though not on Dan Rather's news show.

All this is designed to disguise a horrible fact: (We are about to lose our Walter Cronkite.) Since April 16, 1962, he has anchored the news on CBS, leading in the ratings since 1968 and launching the first half-hour nightly newscast on any network in 1963 by interviewing President John F. Kennedy.

We have learned little about the man, really, in all those years and it's probably true that we just don't want to know everything. We don't really want that superhuman and yet comfy quality to be compromised by any annoying realities.

Cronkite is known around CBS as something of a prima donna. It's been thing of a prima donna. It's been said he demands that he appear on camera for a certain percentage of each night's newscast. From time to time he may have given the impression to colleagues that he owner the big stories for himself because the public has voted him pope. "He's the 500-pound gorilla who always gets what he wants," one CBS News producer has said.

TV Guide once reported that Cronkite has been known to doff his jacket at parties and demonstrate Greek dances. A former CBS comrade ways Cronkite not only likes to go out tippling with the boys, he upon occasion leaves them with the check. Former NBC News president Reuven Frank once jokingly suggested that Cronkite's beloved yacht be rechristened the "Assignment" so that whenever the announcer said "Walter Cronkite is on Assignment," the description would be accurate.

There are lots of stories. But none of them has much to do with the National News Uncle whom Americans grew to love and trust: this beacon, this deacon, this icon, this Cronkite.

He was there when he walked on the moon.

He was there when President Kennedy died.

He was there when the hostages came home.

Somewhat along the line, Walter Cronkite became his own best credential.

No wonder CBS and Dan Rather in particular are scared silly about what comes next. And no wonder the other networks are gearing up for raids on the CBS News ratings. ABC News stands to gain the most -- or at least needs to gain the most -- from Cronkite's departure. In recent weeks, its "World News Tonight" has fallen decisively behind NBC's "Nightly News" in the competition for ratings. In fact, points out an NBC News spokesman, since September "Nightly News" has outrated "World News Tonight" for 19 weeks. One week NBC tied ABC, and it lost to ABC only three times.

At ABC News, they're not sure why their evening newscast, which earlier last year seemed to be overtaking NBC for the No. 2 spot, has fallen behind. Privately, insiders blame the low news ratings of some of the ABC-owned affiliate stations in major cities like Chicago and Los Angeles.The off-screen antics of anchor Max Robinson, who recently accused ABC of racism in assigning reporters to inaugural coverage and then awkwardly withdrew the charge, can't have helped.

Anyway, ABC will be celebrating Cronkite's departure in a big way. Anchor Frank Reynolds, an Old Reliable of TV news himself, will write a salute and deliver it on the March 6 edition of "World News Tonight." Reynolds and ABC News earlier offered a touching adios to Cronkite at the conclusion of the Democratic National Convention last summer; ABC even had a shot of Cronkite holding forth in his CBS booth.

In addition, ABC plans full-page newspaper ads in major cities saying "Goodbye, Walter" and featuring a photo of Cronkite puffing on a pipe. Ostensibly this is a way of paying tribute to a celebrated and revered journalist. But it's also a way or reminding viewers that come Monday, March 9, they won't be able to curl up on Cronkite's lap any more. Frank Reynolds has a lap too, the ads will subtly imply. Of course everyone thinks Dan Rather's lap is made of stainless steel. Strong, but hardly cuddly.

"Those people are going to be switching dials," says an ABC News spokesman. "Obviously this is an opportunity for us."

At NBC News, it is expected that anchor John Chancellor "will say something" the night of the Cronkite farewell, but it hasn't been decided what or how much Chancellor will say.

Veteran correspondent David Brinkley, who with Chet Huntley once ruled the news ratings the way Cronkite has since 1968, wanted to interview his colleague for a piece on "NBC Magazine," the most unwatched prime-time show in television.

Cronkite told Brinkley at a Washington party that he'd be glad to do the interview. But only recently, CBS News president William Leonard called Brinkley to say the interview was off, according to an NBC News spokesman. "Yeah, we said no to that," confirms Sanford Socolow, executive producer of "The CBS Evening News."

In fact, Brinkley says Cronkite personally indicated on three separate occasions that he would do the interview, and then Leonard put the kibosh on it.

"Leonard was very ambiguous about it," Brinkley says. "He said something about 'competitive' reasons. When I said I wanted to air the interview the same night that Walter said goodbye, Leonard said, 'that night of all nights I would not want him to do it.'

"I must say I wondered about it." Brinkley has never been known as Cronkite's biggest fan, but he only wanted to do a respectable little piece. "I certainly was not going to rough him up on the air," Brinkley says.

Now Brinkley plans no televised farewell to Old Ironpants (one of Cronkite's nicknames at CBS). But he does have his own theories about Cronkite's success.

"I think it's the way he looks," Brinkley says. "He has that nice pink face, white hair and white mustache. He really looks like your uncle. He looks friendly, he never says an ugly thing. He's nice.

"He doesn't do the news any better than anyone else, so it can't be that. But people are attracted to him. And he has given the news on television a kind of homey quality it has never had before. I think Walter certainly had a hand in that." Is that good? "Yes -- I think so," Brinkley says.

Obviously CBS wants to limit the amount of attention being paid to Cronkite's exit from anchordom -- as much an exile as a voluntary stepping-aside, according to at least one knowledgeable industry insider. Cronkite was by no means ready to retire from his post and go into emeritus orbit, but CBS had to give ambitious Rather the anchor job or risk losing him. And in keeping Rather, of course, CBS sacrificed Roger Mudd, who stomped over to NBC News in a fit of peevish pique.

Behind Cronkite's grandfatherly persona, then, there's a no-joke and high-stakes battle going on. CBS News is not at all sure how well viewers will adapt from Cronkite's celebrated avuncular coziness to Gunga Dan's clipped authoritative martial-arts approach.

"They are sweating it," says a competitor. "They're damn nervous about it." And another notes: "CBS is not getting the 30 percent audience shares they were getting two years ago; now they're down to 27s." There is even evidence that CBS News executives ae discouraging Cronkite from getting too carried away with his own farewell remarks. Socolow indicated that CBS would prefer no long goodbyes. "It's being discussed," Socolow says. "I'm not sure we will arrive at a final determination until the day we go on the air."

One thing, however, has finally been determined: the new color scheme to show Dan Rather at his best. On March 9, the warm, orangey background so hormonious with Cronkite's flesh tones will give way to a sterner, cooler bluish-gray, whipped up in the lab to flatter Rather.

Going from Cronkite to Rather will be a little like following Ronald Coleman with Clint Eastwood. It's going to take some adjustment.

We think of Walter Cronkite as having always somehow Been There -- offering a cup of hot tea to soften the impact of a day's cold realities -- but in fact, as a competitor points out, Cronkite was behind in the ratings for years before he took the lead, and his importance may be more as a presence than as a bold or innovative broadcaster or reporter.

There were even times when CBS and Cronkite seemed sick of each other and ready to split up. During one election year's coverage of a Democratic Convention, Cronkite's bosses became irritated over his use of the word "erosion" in referring to delegate defections and gave him so much grief over it that Cronkite upped and quit.

"He just blew his top," recalls Socolow. "It was a classic blow-up." Eventually executive Gordon Manning placated Cronkite and convinced him to return to the anchor booth.

In 1978, according to one former CBS News insider, CBS thought Cronkite had grown so bored and weary and was making so many mistakes on the air that it was hoped in some executive suites that he'd retire. But he bounced back and handled his last election as anchorman with his singular, if loquacious, aplomb.

Even when he'd misidentify Sen. Edward Kennedy as "President Kennedy," or even when his voice went so hoarse during marathon coverage that he sounded like Satchmo, or even when he tended to drone on and on like a bionic chatterbox, there was always something commanding and reassuring about Walter Cronkite.

On Friday, the "CBS Evening News" may attract its largest audience ever, and the biggest story may be the newsman himself. On the following Monday night, the network expects a large curiosity tune-in to see Rather's premiere. But sources at the other networks point out that Barbara Walters got a big curiosity tune-in, too, when she debuted on ABC's "World News Tonight" a few years ago. The curiosity lasted for half an hour. The next night it was back to the old standby.

Now the old standby himself will be gone.

How will we say goodbye to Walter Cronkite? Very reluctantly. It won't be easy parting with a man who's come to represent Joe Dimaggio, Billy Graham and Dwight D. Eisenhower all rolled into one.

What's passing away with the phasing-out of people like Cronkite is a tradition in TV news that goes back to pre-TV days of print. Cronkite has said repeatedly that he prizes the early experience he got as a newspaper reporter. Today many so-called reporters in TV news are promoted to top spots and huge salaries on the basis of their cute kissers, not their credentials or their years in the trenches.

The trenches aren't as fashionable now as they were when Walter Cronkite was still wet behind the ears.

Cronkite is one those die-hard sentimentalists who sees journalism as something of a mission.Local stations throughout the country have their clonish white-haired anchormen trying to sustain the illusion of credibility. With Cronkite, it didn't come across as an illusion. We felt we had every reason to believe him.

You could think of Eric Severeid as pompous or remote and still be sorry to see him step down from the Cronkite show, with a few last futile words of caution for the world. You can think William S. Paley may have put a lot of shameful slush on his television network and still know deep inside that when he goes, something and proper and civilized about broadcasting will go with him.

After a speech in Washington late last year, Cronkite was approached by a fresh young reporter with a few questions. "Welcome to the force, son," Cronkite said grandly in his elder-statesman voice. The force is about to be critically depleted by one, and we're going to be losing more than a friendly, pink-faced uncle with a white mustache. In a world that no longer seems to prize gentlemen or scholars, Walter Cronkite did seem a much-needed link with the past, with values and standards that go back further than last week's Nielsens.

When Cronkite says ". . . and that's the way it is" for the last time, it'll be more than the way it was. It'll be the way it never will be again.