One political media wizard calls Harry Muheim "every media consultant's dirty little secret," a description that draws a small laugh from the man who wrote Jimmy Carter's election-year slogan. "Dirty little secret?" asks Muheim. "I'd rather think of myself as their strong right arm."

That's a rare moment of self-congratulation for Muheim, considered by Washington insiders to be one of the best writers of campaign commercials. Muheim, usually wry and self-effacing, is the author of Carter's campaign slogan, "President Carter, a solid man in a sensitive job." And though his name isn't well-known, he's been the pen behind the words of such politicos as former Democratic National Committee head Lawrence O'Brien and presidential candidates Robert Kennedy, Hubert Humphrey, Edmund Muskie and, now, Jimmy Carter.

"The president's slogan emerged in an editing session last November when we were putting together the first Carter spots," says Muheim, a 60-year-old Washingtonian who also writes fiction, sceenplays and other projects between campaign spots. "Jerry Rafshoon had been plugging for 'it's a matter of value,' and that's been used a couple of times. But we were just searching around for another phrase that . . . encapsuled things a bit. One of the important things about this kind of writing is to be slightly unexpected. Most political rhetoric is so totally expected that people keep falling asleep."

Muheim's fans, such as filmmaker Charles Guggenheim, say Muheim is fast, that he can watch miles of videotape of a candidate and quickly devise a theme and structure for an ad campaign. Muheim isn't overly impressed with that talent.

"I think of myself as pushing back the frontiers of human ignorance in 30-second bursts," he says. "Humphrey once rejected a 60-second spot I did, saying, 'Harry, you've not solved the problem of the great American tragedy of Vietnam in 55 seconds.'"

This season Muheim might have a chance to duel an old foe. In 1966 he wrote campaign commericals for Pat Brown's bid for the governorship of California. Brown's opponent: Ronald Reagan.