Mark Russell breezes into his small, cluttered office down a corridor off the main lobby of the Shoreham Hotel, wearing a lime green down jacket over a tangerine shirt and lemon yellow Lacoste sweater. Very laid-back L.A., which may be a portent of things to come. And he doesn't even play golf, he admits. Not yet, anyway.

Russell is finishing up his final three-week stand at the Shoreham. His final show is tonight at 11. With that show, Mark Russell closes out a 20-year run as resident political wit, and a rather remarkable chapter in Washington nightclub history. Twenty years of poking fun at presidents, senators, congressmen and other political figures -- everybody from JFK to Reagan, and definitely not excluding Nixon, Ford and Kissinger, and the Watergate gang -- have made him, like it or not, a kind of Washington comedy institution. Russell, to his everlasting credit, still tends to wince at words like that.

But his presidential one-liners, famous long ago, are Washington's own memory lane of irreverence. Examples:

"Everybody who served with JFK on PT-109 was given a tie pin. That boat must have been bigger than the battleship Missouri."

"LBJ is working too hard. Last night, he shook hands with his beagle and picked Hubert up by the ears."

"Richard Nixon's resignation made me very sad. I had to go back to writing my own material."

"Gerald Ford reminds me of the guy who answers the meat buzzer at the A&P."

"Jimmy Carter . . . I worry about a man whose teeth look like Chiclets."

The Republicans, of course, are his favorite targets these days:

"Take a look at this band," he says, pointing to the Ramone Balve Trio behind him in the Shoreham's Palladian Room. "That's what the National Symphony will look like after the budget cuts."

He says of this administration's style, "This crowd makes the czars look like Ma and Pa Kettle . . . $209,000 for china," he shakes his head. "There's a lesson there. The good people eat off the plastic dishes, and the plastic people eat off the good dishes."

"Go figure Richard Allen taking $1,000 from the Japanese," he tells his audience. "I think it was sneaked to him by Al Haig wearing a kimono. That man Haig . . . I call him Al, he calls me son of a -----. But he's got style, he's got panache -- he went to the Philippines and waded ashore."

Later, Russell finishes jauntily, to much laughter and applause.

He is ready to do some reminiscing. Braced, you might say, as he pours himself a glass of vodka and plops down on a worn, brown-plaid sofa, looking ruddy and buoyant and sort of ageless for a man who'll turn 50 next August.

He has good reason to look buoyant. His career has never been in better shape, what with the PBS half-hour comedy specials emanating from Buffalo, N.Y., Russell's home town; the regular stints on NBC's popular "Real People" series, and the recent, really-went-over-big appearance on the Johnny Carson show, with Joan Rivers as host and Tony Orlando also feeding him straight lines ("They asked me to come back," Russell says and, clearly, he intends to). Then there was the Gerald Ford Museum Dedication special from Grand Rapids where host Bob Hope, long a Mark Russell booster, gave him a verbal pat on the back ("Awful good, Mark, awful good") as Russell finished his segment before a glittering audience that included President and Mrs. Reagan, who also seemed to enjoy Russell's zings and arrows. Not to mention all the one-nighters from here to Peoria and beyond, for which he is booked solidly now through most of '82, with some bookings into '83.

He does seem to thrive on the one-nighters. "I still enjoy it, if you don't run 'em too close," he says. He does about 80 to 100 a year. "It's been running twice a week -- conventions and chambers of commerce and schools, most of them smaller colleges. The smaller the college the more they prepare, the happier they are to see you. I played Harvard last year. I'm playing Yale. At Harvard, I did it for the Kennedy School. They put you up in Kennedy's freshman dorm. They tell you things, you know . . . if those walls could talk, right?"

His younger brother, Dan Ruskin, is Russell's manager. And his wife, Ali, is the bookkeeper. "It's sort of a family act, you know," Russell says. "Everybody's involved." His regular fee for personal appearances is $4,000, and there's also the income from the Shoreham -- about $70,000 a year -- and the TV, and a syndicated column. He had to drop the radio show; he says it was too tough to do from the road. Does Ali accompany him on the road? "She didn't go to Emporia Kan. , but she did go to Palm Springs," Russell replies. "You figure it out."

As for leaving the Shoreham Hotel, the hotel that helped make him a household word from Annandale to Anacostia, he would like to clear up some misconceptions. It wasn't a snap decision, or something born out of lingering hard feelings. "I decided last spring, in April, when 'Real People' picked me up again, for the third season, that it was time to go," he says. "I had been giving them less time as the years go by, anyway. In September I was out on the West Coast , I mentioned to George Schlatter producer of "Real People" -- he's thinking about doing a satirical show -- that I would love to host something like that, and he said, 'That's great, great, I'd like to get you out here . . . Oh, by the way, we go to New York next week to do that stuff at the Stock Exchange . . ." Russell laughs. "It's that casual. If you're there, you're on their minds a little more, you cross their paths a little more . . . You just sort of thrust your body in front of them.

"The problem is not with people," he continues. "It's in my head. I've really had it with hotels, and I just don't like the nightclub thing anymore. I'm not that comfortable between shows, for example. I get spoiled out there, in a hall that seats 2,000 people. You go on, you do your show, and you're finished for the evening. But I couldn't have done it without this the Shoreham nightclub format . This has been the most lasting thing in my life, absolutely. I don't want to leave it other than under the most pleasant of circumstances . . .

"If I had left for good in '78, when I stomped out because the hotel's former owner, not understanding Russell's appeal and underestimating his popularity, wanted to replace him with a disco act . . . I've only worked here six months out of the year since then. It's really phased withdrawal, without the withdrawal pains." No ceremony? No champagne and cake, no speeches, no tears? "No, no, absolutely not," he says. "Well, nothing fancy," he can't resist amending. "Archbishop Hickey and a dozen altar boys. Walter Fauntroy is going to sing one song, and that's all . . .

"I don't want to turn my back on the Washington audience. They've always been the best audience, absolutely."

He will do the White House TV correspondents' dinner in March, and he will come back in the fall, most likely to Ford's Theatre for several performances. "I'm exploring the one-man show format," Russell says, "something with props, visuals, maybe music.

"I'll keep the Washington frame of reference," he adds. "But the Washington reference is pretty well established with the general public now. You just take the bread-and-butter issues and drain the satire, and there's no problem."

Russell and Ali will keep their condominium near Washington Cathedral. The oldest of Russell's three children from a previous marriage, Monica, 22, a senior at Catholic University, lives with them.

"I'm not pulling up stakes," he says. "I'll just go out there and rent a place, for the summer at first . . . I'm not dazzled by all that. If I were 22, I might be. I'm going to test the waters, with the knowledge that what I've done has been accepted, to a degree.

"I can always pick up this again," he adds.

Is money important? "I'm pretty conservative with money," he says. "There's a caution because of the Depression, that was laid on us as children -- certain things that you've heard, or little miserly truths that stay with you . . ." Russell laughs softly. "I talk to so many people about it. I will only drink a little glass of orange juice in the morning . . . "

The hotel, not surprisingly, hopes he will change his mind, or at least modify his plans. It doesn't seem likely. Not even a Willard Hotel (the butt of many a Mark Russell joke over the years) restored to Plaza-like magnificence could lure him back into a hotel lounge at this point. "I don't know about the Shoreham," he says slowly. "They come back to me with proposals, weekends, this and that. My calendar is full -- '82 is gone. I couldn't come back even if I wanted to. We're going to Australia the next day tomorrow ," he adds. "I'm going to do a commercial. It's for an Australian vitamin, and they need an American . . ."