Chris Elliott's impression of Franklin Delano Roosevelt leaves everything to be desired. Except laughs. "F.D.R. -- A One-Man Show," premiering at 11:30 tonight on Cinemax, is patently ridiculous and perfectly insane, a half hour of loony lampooning, absurdist art and delicious mischief.

Looking vaguely like FDR, but sounding nothing like him -- given in fact to Teddy Rooseveltian interjections of "Bully!" -- Elliott is out to bury the whole concept of a one-person show under a thick blanket of farce. If only it proves thick enough, one might hope, to smother the genre once and for all.

Elliott, who cowrote the piece with director Matt Wickline, takes a small but attentive audience at the Irvington, N.Y., Town Hall Theater through a condensed edition of FDR's life and times, except that history has been liberally embellished to up the entertainment ante. In this version, FDR does rope tricks from his wheelchair (with Elliott doubled by another actor who's not even wearing the same color suit), flashes back to fanciful donnybrooks and knife fights with mom and sits out two years of World War II shipwrecked on an island after the White House has been bombed by the Japanese, or maybe the Germans. Maybe even the Swiss.

What Elliott and his cohorts have fabricated is the ultimate excruciating night in the theater, replete with laborious examples of stagecraft-as-soulcraft, that can be safely relished from the other side of the picture tube. Relished, indeed, since some of the jokes only register on the second or third viewing. And others perhaps only on the 14th or 15th -- but we're theorizing now.

Anyone ever trapped in an auditorium with a sole posturing actor holding forth on stage will suffer painful jabs of recognition, but Elliott goes farther than satire, crossbreeding genres and venturing into the outer limits of farce. The performance includes an intermission in which the audience is invited to walk about onstage while the star remains in character. This leads to inevitable unpleasantness and also to a cameo by Elliott's brilliant father Bob, half of Bob and Ray and something of a good luck charm since he appeared in his son's last Cinemax special, "Action Family."

Next door to the theater, the local basketball regional finals are in full dribble, and the assembled throng can't be blamed for its enthusiasm when the victorious team sprints into the playhouse, heralded by good-sport sportscaster Marv ("Yes!") Albert, who hosts "F.D.R." from a box seat. He is joined there briefly by the ghost of Abraham Lincoln, which always seems to show up at these sorts of affairs.

The audience certainly showers Elliott and his conceit (in both senses of the term) with proper deference. They cheer like teen-agers at the show's rock-concert opening, light matches to plead for an encore, and laugh at such dubiously witty examples of the Roosevelt Wit as, "Well, my good man, to my calculations, if there was a war between the chickens and the pots, I dare say the chickens would win!" From start to finish, this is deeply funny nonsense.

Cinemax has the best, or at least the most eclectic, comedy in cable (of course, all of cable seems a comedy at times). Now that he's spoofed the one-man show, Elliott might consider savaging one-comedian specials, like this one. So what if it's a helpless victim; that seems Elliott's specialty. He's a very enterprising nut case, and "F.D.R. -- A One-Man Show" is one gigantic cashew. Gesundheit. Bully!