Robert Klein stands on the exact dividing line between two generations of stand-up comedians. On one side are toastmaster-quipsters like Bob Hope and Johnny Carson, who maintain an attitude of genteel decorum as they tweak human foibles. On the other are brilliant brats, from Robin Williams to Eddie Murphy, who have re-invented the image of the comedian, as a rude and raunchy surrogate rock star wallowing in cultural trash. At his most incisive, Mr. Klein partakes of the best of both worlds without actually falling into the gutter.
''An Evening With Robert Klein,'' which concludes its three-night run at the Circle in the Square Theater tonight, begins with courtly stand-up humor and ends with rock-and-roll self-parody. Over the course of the show, part of the First New York International Festival of the Arts, the 46-year-old comedian runs a continuous obstacle course as he stalks around the set of the ''The Night of the Iguana,'' the Tennessee Williams play currently in residence at the theater. Forced now and then to pay attention to the terrain, which he negotiates smoothly in red-striped running shoes, Mr. Klein gets off some nifty spoofs of Williams dialogue.
But the essence of Mr. Klein's humor, which hasn't changed drastically over the years, blends a clear-eyed puncturing of social hypocrisy with a current of urban paranoia. Mr. Klein, who grew up in the Bronx, is especially sensitive to anti-Semitic implications. And one of his funniest and most famous bits remains his fantasy of ordering a kosher meal on an airplane and being subtly intimidated about it over the loudspeaker system.
One measure of Mr. Klein's solidity is how well so much of his vintage material holds up. His reflections on the old Edgar Bergen show and the ludicrousness of radio as a medium for ventriloquism reminds us that broadcasting has always had its quota of flimflammery. His assessments of recent television advertisements for Chrysler cars and for pyramiding real estate schemes are equally scathing, as they point out the bald-faced chutzpah of the claims being made. But Mr. Klein's swipes at advertising are part of a deeper, understated vision of the way advertising corrupts everything, including history. After giving a touching thumbnail biography of George Washington, Mr. Klein reminds us how the name of America's first President has become synonymous in most people's minds with holiday mattress sales.
Where the younger generation of comedians revels in absurdity, Mr. Klein takes a longer, more literate overview of the state of the world. ''I don't think things today are worse than ever,'' he insists, citing the illustrated text ''Wisconsin Death Trip,'' which documents the desperate lives of turn-of-the-century Midwesterners, as evidence that tales of America's golden past are specious.
His scrutiny of cultural values culminates in his soft-rock musical parodies performed with an instrumental trio and two backup singers. Adopting a soulful Southern drawl to sing a blues number at the end of the show, he breaks off the song to ask a question, ''Yes, I'm Jewish - why do I sing like this?''
The raising of such questions has made Mr. Klein one of the most influential spokesmen for a kind of common-sense absurdism that remains the hardiest genre in the topsy-turvy world of stand-up comedy.