Washington — Stephen Winchell says many of the people who buy grapefruit by mail are Republicans. He knows this because he can persuade them to spend money. Mr. Winchell's job is sending strangers letters that ask for contributions to the GOP. Among his favorite sources of names are customer lists from mail-order fruit companies.
''Ruby Red grapefruit buyers,'' he sighs; ''that list has always worked.''
Roger Craver believes one of the first-read parts of his letters is the postscript. He makes them as direct and personal as possible. Here's one Craver effort: ''P.S. ... Please consider that we have JUST 30 DAYS LEFT when you write your check.'' It is signed ''Geraldine Ferraro.''
''What we're creating here is the impression of a hastily done letter on an airplane,'' says Mr. Craver, a Democrat. The letter was actually printed by computer.
Winchell and Craver are pouring millions of dollars of other people's money into politics. They are two of the foremost American practitioners of political direct mail, the difficult art of raising cash - and candidates' profiles - via the United States Postal Service.
Direct mail combines the emotional appeals of a revival meeting with the high technology of the latest electronics. It is, say experts, one of the fastest growing and least understood of modern vote-getting techniques.
It is becoming the main means of fund raising for political parties. It can help make dark horses front-runners for presidential nominations (Ronald Reagan and George McGovern, for instance). It is used in at least 50 percent of congressional races.
Direct mail is based on the theory that people will give any piece of mail at least a cursory glance as they walk toward the nearest trash can. (Research shows people read 80 percent of their mail while standing up.)
To keep their missives from being pitched unread, direct mailers such as Winchell and Craver try to send messages ''almost as much fun to go through as a box of Cracker Jacks,'' according to Larry Sabato, a University of Virginia political scientist.
One current letter from the Republican National Committee includes a touching photo of President and Mrs. Reagan, waving and ruddy as they embark for Camp David. Other popular enclosures include party membership cards, ersatz political memos, questionnaires, cassette tapes - everything but RNC decoder rings or locks of Walter Mondale's hair.
The letters are as personalized as possible. A particularly popular approach right now is the candidate-spouse letter, a homey message that ends with a soft-sell appeal for funds. Craver recently sent out a piece of Democratic direct mail on Joan Mondale's personal stationery. ''For more than two years I've traveled across America campaigning for my husband,'' it begins. ''I hope that Fritz and other Democratic candidates can count on your generosity in these crucial, final weeks before election day.''
Such a simple approach can result in a big payoff. In 1980, one famous letter garnered John Anderson 250,000 donors - almost as many as Ronald Reagan had listed after six years of direct-mail fund raising. In 1972, George McGovern milked direct mail for $12 million in quick contributions.
Whom you send your message to, of course, is the key. Developing these mailing lists for a candidate or party is called ''prospecting - reaching out into the gravel bed of humanity and finding the gold nuggets,'' Craver says.
At the prospecting stage you're lucky to break even, he adds. Democrats prospect among the subscription lists of The New Republic, The Nation, and other liberal publications; membership lists of groups such as the Sierra Club; and various Democratic voter lists. Republicans mail to National Review subscribers, business groups, and the well-developed in-house lists of Republican Party committees. It is with these house-list mailings to previous donors that candidates can make the money, getting back up to $4 for each $1 investment. People who donate to a party or candidate can thus expect to receive eight or 10 follow-up fund-raising letters.
It was conservatives who first tapped direct mail on a lucrative scale, and who continue to lead in the politics-by-Postal-Service field. Modern direct-mail techniques can be traced to the 1972 presidential bid of George Wallace, according to Roy Adler, a political scientist at Pepperdine University. (Conservative activist Richard Viguerie designed the Wallace mail effort, Mr. Adler says.) Barry Goldwater raised one-third of his money for his 1964 presidential bid from direct mail.
Today the Republican National Committee has 2.5 million people on its house direct mail list and raises some 70 percent of its funds by post. For this Winchell is largely responsible. His company, Stephen Winchell & Associates, set up the GOP direct-mail machinery and continues to do much work for Republican committees.
This year Winchell will send out 100 million pieces of mail. They will bring in an estimated $70 million for the RNC, other GOP panels, Reagan/Bush '84, and a few individual candidates.
Throughout the early '70s, the GOP raised funds under the theme ''Let's preserve the two-party system.'' That, says Winchell, was not the sort of slogan that caused people to reach for their checkbooks. After President Carter took office, Winchell roused Republican direct mail to the attack - centering appeals on specific items such as President Carter's budgets and the Panama Canal Treaty. ''People give for issues,'' he says, ''and basically we brought issues to the Republican Party.''
On the other side of the political spectrum, Craver agrees that direct mail works best when it's used to fight for a cause. A committed Democrat and a sort of interest-group entrepreneur, Craver has for the last 10 years used mail appeals to help build the Sierra Club, Common Cause, and other citizen groups into political powers.
Today Craver's company is behind the Democratic National Committee's push into direct mail. Since 1981, Craver, Mathews, Smith & Co. has built the DNC's house list from virtually nothing to 650,000 mail donors.
''Direct-mail fund raising works best on the polar points of the ideological spectrum,'' says Craver. ''Donors are issue donors, not candidate specific donors.''
As a result, direct-mail appeals tend to be strident. They picture opponents as people who may destroy democracy as we know it if a $25 check is not in the mail within the month. (Sen. Edward Kennedy is a favorite GOP ''devil''; Sen. Jesse Helms serves the same role for Democrats.)
The technique can also be misused. Mr. Viguerie has likened direct mail to a snake - something that is silent and deadly, an easy way to disseminate misleading information about your opponent, without his or her knowing.
Even with legitimate material, ''it is a wonderful means to get out your story with your opponent never understanding the magnitude of who, and how many people, you're writing to,'' Winchell says.