ROBERT KLEIN is upset. Just landed in Los Angeles, he has spent the last half hour on the phone in his Beverly Wilshire suite talking to his wife, opera singer Brenda Boozer, who has lost her credit cards--their credit cards. "Luckily, they know me here," Klein sighs, sitting in the hotel bar. "They know I'm disreputable."

"I guess she won't be doing the American Express commercial for a couple of weeks," he deadpans, grabbing a handful of peanuts from a bowl on a table.

Actually, the 38-year-old comedian seems quite nonplussed. He is in L.A. for one of his frequent appearances on The Tonight Show, this time as guest host. What does he think about Johnny's new shortened one-hour format? "Well, I think they kept it to ninety-minute money," he grins.

Ah, money. Not to suggest that Robert Klein, former middle class Jewish Bronx kid (he's still Jewish, but no longer middle class and living in the Bronx), is in need of the bucks, but earning a living seems to be one of the reasons Klein has flown West. "After a wonderful summer I was rudely awakened to open at Harrah's in Lake Tahoe. Two weeks in paradise. Thank you! Quite enough!

"While I don't like performing in places where the audience is wining and dining by the thousands, I must say that Harrah's pampers its artists. I had the largest hotel bill I've ever seen--all free! They picked up all my incidentals, all my phone calls, everything. I've never been treated so royally."

Does the entertainer find much difference between playing Tahoe and Vegas? Is there more audience familiarity with his work at one or the other?

"Well, I only played Vegas once, in 1972. But the two places are interchangeable--like Ricardo Montalban and Fernando Lamas."

As a stand-up comic, which Klein takes great, immodest pride in being, he much prefers the college concert circuit to the big money gambling and entertainment dens. "Where they don't serve drinks or food. With theater seats, facing one direction--college gymnasiums, lecture halls, theaters. With college audiences I can reach for the highest.

"Stand-up hasn't even been dented yet. I was so tremendously inspired by Lenny Bruce and Jonathan Winters. That combination, to me--and a little bit of Rodney Dangerfield--that combination is the consummate."

Dangerfield's is a name that pops up a lot in Klein's conversation. The pair worked together during the fledgling days of the Improvisation club in New York. "Rodney helped me a lot," Klein says, affectionately. Another familiar name from the Improv days is Richard Pryor; despite their considerably different backgrounds, Klein says the two shared a similar comedic sensibility and perspective on life.

The affably acerbic Klein is also a veteran of Chicago's much esteemed comedy training ground, Second City, where he worked with the likes of Fred Willard and David Steinberg. "David was mean. Inconsiderate. A pest and a twirp." (Klein seems to be reveling in the sheer delight of his venom, more than really meaning what he says.)

Since his early days, Klein has enjoyed a varied, successful show biz career. He has done Broadway: first, a small part in The Apple Tree; his latest, the lead role and a Tony nomination in Neil Simon's They're Playing Our Song. He has made records; out-of-print classics like Mind over Matter and Child of the Fifties and, upcoming, an all music and song collection for Casablanca (to be followed by more comedy discs). He is oft-seen on television: HBO shows, Johnny Carson, and two NBC specials this fall, with guests Rodney Dangerfield, Jane Curtin, Judd Hirsch and the Charlie Daniels Band. And he is not unknown in movies either: The Landlord, Hooper and the in-progress animated film The Last Unicorn. Klein's also got some of his own film projects and scripts on the burner.

One of the closest things to Klein's heart, though, is his radio program, The Robert Klein Hour. Twenty-six shows aired last year, syndicated nationwide by the FM outfit responsible for The King Biscuit Flower Hour. Top name rockers like the Who, Fleetwood Mac and Carly Simon came on and talked with Klein (and also played their records), as well as non-music folks like Rodney Dangerfield (yet again), Robin Williams and Jane Fonda (who didn't play her record). This year, Klein's own production company has taken over the show, with a resultant change of advertisers.

"Instead of Budweiser delivering seven cases of beer a week to my house, Faberge is my new sponsor. I drink only Brut on the rocks and organic shampoo--wonderful.

"Radio is peculiarly well suited to talk shows," he observes. "That is, if you have a guest who can talk. William Shatner and Persis Khambatta came on to plug Star Trek--The Motion Picture. I didn't see the film. They didn't want the press to see a screening--the assholes.

They made a strategic error with Shatner. They had Khambatta and Shatner come up together as co-stars of this movie, when he's a major American star and she's an unknown.

"I couldn't prove it, but I'm pretty sure that was the reason he was so uninspired. And if he's not inspired, why should I be? I'm trying to pump his film for him, right, so he's answering 'Yes' and 'No' like Broderick Crawford. 'So, really,' I say, I understand the effects cost several million dollars?'

"'Yeah.'

"So, Budweiser's the sponsor, right, so I say, "Tell me this (adopts gruff Bronx street accent and yells) 'ANOTHER COLD ONE, MISS KHAMBATTA? TWO MORE COLD ONES FOR MISS KHAMBATTA. HEY, BILLY, YOU WANT ANOTHER ONE HERE? SO, BILLY, TELL ME, IS THERE ANY BEER IN OUTTA SPACE?' He was awful."

Despite all the jokes, like most of his colleagues Klein takes his work most seriously. One senses the frustrations and ambitions that lie just beneath the casual demeanor. The concerns with the power structure of Hollywood. And concerns with another kind of power structure: government, politics. Though he has fairly bandoned the political humor and satire that was so much a part of his repertoire during the Watergate era, he hasn't stopped caring.

"I'm still a political person," he asserts. "I played at the White House a year ago and I got a standing ovation. I invited the President to jump onstage to see what a standing ovation looked like."

With the nation on the precipice of choosing between Carter and Reagan, who is Robert Klein, the sagacious, articulate philosopher/comic supporting?

"Carter. First of all, he invited me to the White House--I have to support him. And," he pauses, clutching some more salted cocktail nuts, "he offered me his sister."