"Don't you understand the rather comic dimensions of it all?" Cash (Duffy Epstein), the martini-swilling plastic surgeon, asks Mr. Hadid (John San Nicholas), a vaguely Asian cab driver, in the midst of explaining how his family happened to accidentally cause the death of Mr. Hadid's wife. "Look, you want to be more like us. But we're a bunch of assholes."

He got that right. Bruce Norris' brutal denunciation of monied liberal America is a comedy so bleak it ceases to be funny. Aptly set at Thanksgiving dinner in the sort of house where dining rooms have multiple levels, it is populated by a family of well-heeled, college-educated, sweater-clad weasels—the surgeon; his 23-year-old Russian girlfriend, Kalina (Amy Beth Frankel); his PBS-obsessed mother, Carol (Jacklyn Maddux); his stay-at-home-dad brother, Clay (Damon Kupper); Clay's high-powered lawyer wife, Kelly (Valerie Stevens); and their 4-year-old daughter, Kayla—who exhibit the sort of two-faced selfishness one expects only from the works of David Mamet. That Norris is fully aware of his characters' unpleasantness doesn't make them any more fun to spend an evening with.

Norris is hunting for humor at the brink of tragedy, dwelling in the moments when the evening threatens to fly out of control. I laughed hardest at the potentially sinister origin of Kayla's genital rash, Kalina's explosive incredulity at Carol's endorsement of socialism, and Clay's near breakdown over Kelly's insistence that he euthanize his cat to protect their unborn child from toxoplasmosis. But the dialogue that fills the spaces in between, while sharply written, is tediously mean-spirited. Wealthy leftists happily espouse sympathy for the less well off so long as they aren't asked to make any sacrifices themselves. I get it; I don't need two hours of reminding.

Third Rail's production shows the company's usual efficiency and polish, handling the show's many temporal changes and myriad props with aplomb, even as the script's negativity grates. The sole relief in the mire of meanness is Frankel as Kalina, the transparently emotional, brightly sexual and delightfully impulsive Russian émigré. She's a twit, but at least she's an optimist. With this crowd, you take whatever sliver of joy you can get.

SEE IT: Third Rail Rep at the Winningstad Theatre, Portland Center for the Performing Arts, 1111 SW Broadway, 235-1011, thirdrailrep.org. 7:30 pm Thursdays-Saturdays, 2 pm Sundays. Closes Oct. 30. $29.50-$38.50, $14.50 students and rush tickets.