If you were a southern Democratic in reelection trouble, you might want Marvin Chernoff around to handle the media part of your next campaign. Not all the candidataes who come to him have image problems, but enough to give Chernoff a reputation as a savvy adviser.

Consider the case of South Carolina's Rep. John Jenrette, whose 1976 reelection effort was hindered by a messy divorce. His hometown newspapers reported that when told his wife had listed a couple dozen cases of infidelity, Jenrette supposedly quipped, "Whew! Is that all she got?" And it was discovered that a land-sales venture in which he was involved included some underwater lots.

The smart money predicted the congressman's early retirement, but Chernoff's campaign spots stressed Jenrette's Capitol Hill achievements: "Finally," the ads concluded, "we've got ourselves a congressman. Let's keep him." He wond with 56 percent of the vote.

Then there was Donald Stewart, a little-known Alabama state senator who wanted to win the special election for the U.S. senate seat left vacant by Sen. James Allen's death. After George Wallace dropped out of the race, Stewart faced Allen's widow, Maryon Allen, who had been appointed to fill her late husband's seat until a special election.

"We had to run as James Allen's natural successor against his wife," Chernoff says. "Mrs. Allen didn't come to Alabama to campaign much, which created an opening. We ran against what we perceived as her unresponsiveness."

The 47-year-old Chernoff calls his brand of political assault "postitioning." His theory is that both commercial and political ads have passed through three eras: the product era, in which an advertiser merely had to announce his product and tell why it was swell; the image era, in which an advertiser built a feeling or image around a product to make it desirable; and the era of positioning -- today -- in which the world is so cluttered with information that successful marketing means creating a positive impression in the consumers' and voters' minds.

Chernoff, the former owner of an office equipment store in Cleveland, got into the business in 1967 when he helped a friend, Carl Stokes, become Cleveland's mayor. He worked Ohio for Hubert Humphrey and George McGovern, then decided to live in South Carolina after helping Charles "Pug" Ravenel win a tough primary for the governorship of that state in 1974 -- a victory the state supreme court nullified by ruling Ravenel hadn't met the residency requirement.

"I became obsessed with how important the media was," Chernoff says of the Ravenel campaign. He began his own firm, now called Chernoff/Silver and Associates, with offices in Columbia, S.C., and Washington.

Other clients included Bill Clinton, the young governor of Arkansas; Rep. Ken Holland (D.-S.C.); and the National League of Women Voters, for whom Chernoff produced Equal Rights Amendment spots: "We're not going to say we've won all our campaigns."

Although he's a Democrat, Chernoff fears for the president's reelection chances.

"Jimmy Carter -- talk about positioning," says Chernoff. "He's seen a nice guy who's not terribly competent. Once you have that position in anyone's mind, how do you change it?"