William Leonard sits in the president's office at CBS News in New York and tries to keep CBS News from falling apart. "I've spent my life getting ready to do this job," says the 33-year veteran of CBS, "so even if I hated it, I'd love it."

At the moment, of course, Leonard is occupied also with a world that seems to be falling apart. Yesterday he joined other news executives in sending cables to Iranian officials to protest the country's expulsion of American journalists. The action "is contrary to widely accepted freedom of information principles to which your country subscribes," wrote Leonard in a burst of diplomatic guff.

But many of Leonard's long-term worries are right under his own roof. William S. Paley first built CBS into a powerhouse by coaxing top talents like Jack Benny away from other networks. Now Leonard has to fight star wars of his own as ABC and NBC news presidents Roone Arledge and William Small dangle generous offers in front of CBS News luminaries.

Already the migration has been substantial. Small himself, irked when passed over for the CBS News presidency two years ago, took the NBC News throne as compensation. Wry and dapper Hughes Rudd hot-footed it to ABC, and former CBS News president Richard Salant went to NBC when forced to retire from CBS at the age of 65 -- as much all executives except Paley himself.

Now there's a veritable CBS News alumni club at NBC News, and Salant reportedly spends much of his time sending out cranky memos that make everybody angry. He's used to the way they did things at the old boy network within an old boy network, CBS News.

"Bill Small, Roone Arledge -- they're doing just what I suppose I would do if the roles were reversed," says Leonard. Just last week he lost another pair of workers to NBC News, as chief publicist Ellen Ehrlich and her assistant, Ron Najman, jumped ship to be with their old pal Small.

Leonard in fact does not like giving interviews but gave a few recently because he was tired of reading about Small. There have been reports of at least one major fit thrown in his office because Leonard thought Small and Arledge were getting all the attention.

True, CBS News is still the most prestigious news broadcast organization in the country. True, "60 Minutes" has pulled the magnificent trick of becoming the most popular news series in TV history, often at the very top of the top 10, ahead even of the most deliriously escapist junk on the air.

But the most successful you get in television, the more profits go up (network news is now highly profitable) and the more pressure goes up. Intensified competition from ABC News, which Arledge awakened from catatonia a few years ago, doesn't make Leonard's life any easier.

Nor does it help that CBS News is a colorful menagerie of fragile egos waiting to be bruised. Walter Cronkite turned 63 in November and might leave the CBS Evening News at any moment. Roger Mudd has a legendary temper and threatened to quit when it looked as though CBS might not shoe his documentary on Teddy Kennedy.

In fact all anybody at CBS News has to do these days is mention the letters NBC in that order and executives swivel in their leather chairs. Leonard has been running around trying to sign up everyone but the cleaning lady to new contracts. Yet some people are reluctant to negotiate with a lame-duck news president who, like Salant, will be forced to retire at 65, less than two years from now.

Leonard denies such rampant rumors as the one that has Dan Rather sour and unsatisfied over his role on "60 Minutes" and the fact that he hasn't been named heir apparent to his majesty Walter Cronkite for the dark day when Uncle Walter abdicates.

"Dan has a contract that has a little less than two years to run, which means he isn't halfway through it, so we're talking about a time two years down the road," says Leonard. "Cats can look at kings, and Dan would be less than human if he, along with four or five other people who are extraordinary broadcasters, didn't say, 'I wonder what's going to happen to me when Walter quits his broadcast.'

"But it's not this year's problem. It isn't even next year's problem. It may be the year after's problem." Can Cronkite keep doing the Evening News as long as he wants? "Absolutely, absolutely," says Small (the 65 age limit applies only to executives, not talent).

However, Leonard concedes that Cronkite's enthusiasm for the job "varies from moment to moment and week to week and vacation to vacation. I would guess he will do it just as he's doing it now until he's 65 and then have some other arrangement. There's many things that Walter Cronkite could do here without doing the Evening News every night of the week. Mike Wallace, too -- he's getting along in years."

Leonard denies that there is an appointed successor to Cronkite now roaming the halls of CBS News with a halo over his head -- The Chosen One. "It would be foolish to say that anybody who is prudent about the future hasn't spun the bottle and wondered what would be best. It would be a very important decision and I think abut it all the time, but no one has said, smack, that's the way it's going to be."

How the dominoes fall or stand at CBS News will have a great effect on TV news in the '80s, a decade in which the amount of time given over to news programs on the networks is certain to grow (already Arledge is talking about converting ABC's late-night Iran crisis reports into a permanent newscast when the crisis ends -- if ever).

If ABC News continues its competitive surge and NBC News is pulled out of the Valley of Low Morale where it now languishes, CBS may have to start behaving less like Edward R. Murrow and move like Geraldo Rivera. Or Pinky Lee. If the Arledge Big Top approach succeeds, the other networks will have to scramble to equal the flash and splash. Leonard hopes this doesn't happen. He sees himself as keeper not only of an elite little zoo but also of a scared trust.

CBS News guys are known to be the biggest snobs in the business. But then, there's some justification for that.

"For 50 years, there were people here that felt this was a different place, that this was a place of independence and character," says Leonard grandly. "It always has been, from the day that Ed Klauber came over from The New York Times (in 1930). He was a friend of my father's, and he started this whole business and he was a tough son of a bitch.

"And Paley'll tell you, his standards in those days about news were as high as they've ever been, and he beat it into everybody he hired. That's the only thing they can't hire away. You can't hire people away like that.

"I guess that's a romantic way of looking at CBS News, but it's just exactly a summary of what I believe. I would hope that this would always be the place to be. That's all I care about in this world professionally. That's all I care about. And if it isn't, then I guess I will have failed."

Not surprisingly, Leonard doesn't want to be drawn into criticisms of the competition. But he notes of the other networks' imitations of the smash hit "60 Minutes" that "some are okay, some are awful, some are cheap," and will say of ABC's "20/20" that "the most talented part of it seems to me the promotion (advertisement) of the broadcast within the broadcast.

"I don't know if we have the talent to do anything like that, and I don't think that we want to." Similary, he scoffs at NBC's David Frost-Henry Kissenger fiasco of last year -- "It was mad" -- and says, smiling, "I just couldn't help wondering what Dick Salant would have thought about it if he'd viewed it from here rather than from there."

Salant, though considered king of the stuffed shirts by many at NBC News, is known to have given the Frost deal his seal of approval, much to everyone's later embarrassment.

And so, Leonard is asked, is there a danger that CBS will have to compromise its vaunted principles as the news game gets less gentlemanly, and womanly, in the weeks and years ahead?

"Is there a danger? I guess there's danger everywhere you look that the cheap is going to win and the crummy is going to triumph." This happens, he says, when "the sizzle, rather than the steak, if you will, becomes more important." Is this true of any other network's news operation? "Only comparatively," he says, with what might pass for a twinkle.

As for other aspects of TV news in the '80s, Leonard says, "I find that the future is the safest area in which a fool can throw his weight around," but can be drawn into some prognostication. A one-hour nightly newscast on the network, long a dream of network news execs, is still far off, Leonard thinks, but when one guy does it, all the others will follow suit. The problem remains one of resistance from local stations.

And now for the long view. Come on, Mr. Leonard.

"You look way down the line and I think what's going to change is that box," he says, pointing of course toward a TV set. "That box is going to be able to store and retrieve, and it doesn't matter whether it comes in by satellite, by cable, by mousetrain or what the hell -- people don't give a damn how it gets there, they just want to know what they can do with it.

"What that's going to mean is we're going to have to have a news broadcast that the average person, someday, can say he wants to see at 7 or 6 or 5 -- the 'Walter Cronkite Jr. News,' or whatever I want to see it NOW." And so we're going to have to program different editions. We'll be on the air with different editions of that news or some other news 24 hours a day.

"But when that'll happen, I don't know."