Did you hear that President Reagan temporarily lost his nuclear code card? During that unpleasantness back in Washington. He carries with him a nuclear code card which only the president has in case we get into a nuclear war. And it was missing for awhile. Isn't that kind of scary? Turns out that Nancy found the card and charged three gowns on it at Saks. -- from Johnny's monologue

"Johnny Carson May Be Losing Some Appeal," says a headline in The Wall Street Journal. Not with me, he isn't. The Journal was referring to a decision by NBC's Minneapolis affiliate to delay Carson for half an hour each night so it could air old "M*A*S*H" reruns in Johnny's time slot. Some other stations have pushed Johnny's starting time back to make way for reruns that, says Broadcasting magazine, "attract a younger audience."

How very very cruel.

Johnny returns tonight from his latest lengthy vacation, in time for the February sweeps, amid proliferating reports of his increasing vulnerability. Some see it as a trend, but heaven help us if it is; the idea of going to bed without the hot toddy of Johnny's witty and barometric monologue is a moderately petrifying thought. Johnny has become the national nightcap, a desperately needed swig of merriment with which to end a day, an everymannish comedic ombudsman who makes about $5 million a year but can still get away with jokes about the high cost of groceries.

Johnny just gets better and better; everything else gets worse and worse.

He has probably been funnier longer and more consistently than any other comedian who ever lived; not even the greatest of comics had to hold up through Carson's kind of exposure -- 20 years now, and at least 260 nights a year. A few years ago NBC publicists sent out the word that Carson was the new Will Rogers, but Will would have been hard put to wear as well as Johnny, night after night, for two decades. It's an astounding demonstration of durability, one of the few things in the day-to-day trudge of television to be deeply grateful for.

In fact, though, Carson is not getting the ratings he once got. A few years ago he was regularly attracting 28 percent of the night owl audience. Last year his highest quarterly share was 25 percent; in the crucial fourth quarter, he slipped to 22. He still beats the other networks, especially ABC's high-slippage "Nightline" news show (you get the news in Carson's monologue, too, but with punch lines) and its moldy old reruns ("Fantasy Island," "The Love Boat"), but local stations around the country are throwing big pies at him. "M*A*S*H" pies and the like.

Ironically, there is a new source of competition for Carson: reruns of the original NBC "Saturday Night Live" show, the irreverent humor of which Carson once derided as mean and tasteless. Now late-night "SNL" reruns give him a run for the money (he was approached annually to guest-host "SNL," and always refused). The late-night audience, more youngish than prime time's, may feel that Carson is square while the "SNL" players are terribly hip.

Carson isn't square -- he now does the borderline sick humor he once criticized "SNL" for doing -- but "The Tonight Show" is. It's an abysmally stodgy production, stiff and creaking. When I attended a taping a couple of years ago, I was dismayed by how old and sodden everyone looked; the producer, Frederick De Cordova, should be out whiling away hours on the links.

I visited the program again one night last month. The trappings still seemed pitiful -- the relationship between Johnny and Ed McMahon appears to have reached a testy state that frequently shows up on the air, and they almost never speak to each other during the commercial breaks -- but Carson was as boyishly buoyant as ever.

Some people resent Carson's frequent vacations (15 weeks off a year, plus Monday nights) as if he were an errant civil servant who had refused to do his duty. In 1976, during one of his rare interviews, Carson said he had to take the time off to keep his wits about him. "People say, 'Jeez, Carson's off the show a lot.' Well I'd be in a rubber room and fed by paid attendants if I wasn't," Carson said. "I'm 50 now, and I am sure I won't be doing 'The Tonight Show' in 10 years; I doubt if I'll be doing it at 55."

Carson was 55 last year.

We are obviously laughing on borrowed time.

When you see the show in person, you think, "How trifling this is -- a comedian does a short monologue, then fills out the rest of the hour sitting at a desk and chatting with semi-celebrities." Then you go home and watch the show you've just seen taped, and you realize how perfectly Carson and television go together, a sublime collaboration. No one would go see "The Tonight Show" if it were just a stage presentation. It isn't enough to entertain 500 people, but add television and it becomes more than enough to entertain 15 million.

The show of course is not the thing. Johnny is the thing. "You know what this is like?" Johnny said during one of his January shows. "It's like the challenge of death every night. It's like I'm standing on the ledge of a 20-story building and the crowd is yelling, 'Jump!' " I for one cannot tolerate the thought of life without him.