Once again Gerry and John, favorite couple of pol-watchers everywhere, are experimenting in role reversal. Zaccaro is headed off to serve the public interest. Ferraro is headed off to serve a private interest.

Last week, John was sentenced by the court that convicted him of fraud to spend 150 hours working for the poor. At the same time, Gerry volunteered to make at least $500,000 working for the Pepsi generation. I mean, somebody has to make a living.

What Ferraro is doing, along with her two daughters, is making a commercial for Diet Pepsi. This is not your run-of-the-mill crass commercial. Ferraro's palm will not pat the soda can. Nor will the word Pepsi ever pass her lips. It is one of those classy vignette ads in which the family trio will be seen talking about life and choices for women, followed by a discreet cutaway to the product.

But frame it any way you like, Gerry Ferraro, former candidate for vice president of the United States, is strutting her stuff for Pepsi as surely as Mary Lou Retton is taking a tumble for Wheaties. She has broken yet another barrier and become the first politician to turn to superstar sales.

I do not mean to sound snobbish about this career move. The revolving door of politics has always made for some strange passages. Doing a commercial for Pepsi after you've left public office is no less respectable than joining the law firm that represents Pepsi or becoming a lobbyist for Pepsi. On the other hand, it's no more respectable.

In some ways the turn to truly commercial life is a logical transition. Politicians have some experience with endorsements -- although generally they are on the receiving end -- and they know a lot about advertising. You can't run for office today without learning how to sell yourself on TV. After the election, Walter Mondale's speechwriter, Martin Kaplan, said that only two candidates could have beaten Ronald Reagan: Robert Redford and Walter Cronkite.

But that's what makes the Ferraro sellout so depressing. The relationship between politics and advertising is what you might call intimate. It's hard to separate the sales pitch for a candidate from the sales pitch for a soft drink. It becomes incestuous when the candidates actually sell the soft drinks.

In the last campaign, the ads for the Republican Party were produced by the same guys who made the Coke commercials. They weren't selling ideas, they were selling great American fizz. Today candidates spend more time on their ads than on their position papers. They learn to talk in 30-second bites and are packaged to look like the people who do commercials.

We already have political figures who started out doing ads. The current president did commercial stand- ups for General Electric. Sen. Rudy Boschwitz became known in Minnesota for his company's ads. Lee Iacocca became a possible candidate when he starred in the Chrysler ads. Now we have an ad star who began in politics. It's as if Jimmy Carter started selling Skippy.

Francis O'Brien, Ferraro's campaign press aide, offered another slant to Ferraro's decision to go into the ad pros. He believes that the spot on network television in March will improve her image with the public. In short, he thinks the ad will sell Ferraro, while Pepsi thinks it will sell soda. If they are both right, we will have the first totally merged political and product commercial in America. Diet Ferraro.

I'm just plain uneasy with the way advertising has become the payoff for nearly every achievement in the country. Win a marathon and you get the contract with the shoe company. Go for the gold and you get the shampoo ad. Sing at the Met and we'll give you a shot at the American Express account. Now, if you run for vice president, we'll let you sell Pepsi.

The process turns every achievement into a hustle and every dream into a sales pitch. Now the first woman to run for vice president is cashing in on her achievement. What an odd twist this is to the all-American story of Geraldine Ferraro. The hardworking daughter of an immigrant family has reached the pinnacle. She's going to be on television selling soda pop.