The farm mail's been running only "light to medium" -- 300 letters in five weeks -- in Sen. Nancy Kassebaum's (R-Kan.) office, topped by African relief, Social Security and foreign relations.

The "space mail" to shuttle crew hopeful Sen. Jake Garn (R-Utah) stood recently at 225 letters: 186 in favor, 40 opposed to his joining a NASA mission.

In an institution that trembles with public opinion, it's hard to find a subject too trivial for a mail count on Capitol Hill: Staffers even tallied the letters to Rep. Robert Davis (R-Mich.) over a photo of his wife in a skimpy exercise suit. They reportedly peaked at 50 a week -- no one's saying pro or con.

In 535 congressional offices, the issues may be different, but the gauge is the same: a measure of the clout borne here by every scrap of paper with a 22-cent stamp. The count is not foolproof. It's not representative. Sen. Howard Metzenbaum's (D-Ohio) 6:4 ratio of constituent letters against confirmation of President Reagan's attorney general-designate didn't stop Ed Meese from getting the Senate's nod.

But because constituent opinion is all that stands between legislators and a return to private employ, the mail is treated with respect.

But to what effect? How often does a letter to Congress really have an impact on national policy? Not very often, but there are exceptions.

"Because the mail is mostly reactive," explains one aide, "what happens is it tends to firm up a legislator's resolve rather than change his opinion." Staffers agree its chief use is as a barometer of certain interest groups.

Some suggest the 43 representatives and seven senators in the new congressional class of '84 may give more personal attention to constituent mail now, early in their first terms, before the reality of the workload sets in. At other times, volume, approach or personal influence have made enough difference to affect legislative change. Knowing the system doesn't hurt either.

Mountains of constituent mail -- 200 million pieces were received last year by the House and 41.5 million by the Senate -- are routinely X-rayed, sorted and then turned over to individual staffs to be prioritized, re-sorted by issue (Social Security, balanced budget, school lunch), referred for casework and -- in one of the most back-breaking of legislative tasks -- answered.

"It's the big workhorse task of any congressional office -- trying to move the mail in and out," says Bill Mengebier, administrative assistant to Rep. Davis. "You have to have a system to do it, otherwise you drown in it."

Just trying to appear responsive to a volume of mail that's grown more than five-fold in 12 years could sap a staff's energy. Some Congress members and their staffs go well beyond appearances.

Sen. Kassebaum, for example -- in an act of virtual heresy here -- shuns the ubiquitous Autopen machines and personally reviews and hand-signs each of the 1,000 to 1,500 replies her staff prepares each week. Only when she goes out of town is this authority delegated.

"I don't think it's a very effective use of her time," concedes her press secretary Rob Stoddard, "but she thinks it's important to stay in touch with her constituents and know what they're thinking."

Most can't afford this luxury. What's just possible in an office representing 2.5 million constituents would quickly derail a career in, say, California with its 26 million.

"I don't know any way with 10,000 letters a week coming in you can have your letter get the senator's personal attention," says Roy Greenaway, administrative assistant to Sen. Alan Cranston (D-Calif.). "On big issues, we have to handle it in a mass production way." Incoming mail is matched by issue to any of 1,000 form letters typed in-house -- to protect their mailing lists -- and signed by an Autopen machine.

Fellow Californian Sen. Pete Wilson (R), faced with up to 18,000 pieces of mail per week, relies heavily instead on the Senate's central computer system -- the Correspondence Management Service -- to crank out form letters. CMS laser printers also can sign letters much faster than the old Autopens. Within the next year or so, most Senate offices will have their own computers to handle correspondence and other duties. House staff have access to a choice of computer systems provided by outside vendors.

Most members of Congress rely on regular computerized mail counts to assess public gn can be a powerful weapon. The classic example was the 1982 effort by the nation's banks to repeal legislation requiring that they withhold taxes on consumers' interest and dividend income and send it directly to the Treasury.

Unhappy with the burden this would place on them, the banks mounted a protest in one of the most sophisticated mass mailings of modern times, sending customers, along with their bank statements, pre-printed letters to Congress on personalized-appearing stationery.

Congress was predictably swamped. In the Senate alone, extra help was needed to handle the 1 million pieces of mail a week generated for more than two months.

The maneuver succeeded, but offended many, including senators Dole and Garn.

"We didn't view that as an effective barometer of public opinion," says Dole's assistant Pettit. "An awful lot of people didn't understand what they were writing in about or had their names printed on cards." The banks, says Garn press secretary Bill Hendrix, "created the impression in the mind of the public that it was a new tax that ought to be opposed."

Many lobby groups -- running the political spectrum from the National Rifle Association to the American Civil Liberties Union -- have learned the lesson to avoid a tactic that can backfire.

*""We advise our people to use their own words and stay on just one topic, not to go off on any tangents," says NRA spokesman Andrew Kendzie.

Sally Berman, who writes and edits publications for the ACLU Washington legislative office, says the organization is following the same course in urging opposition to the administration's request for more covert operations funding in Nicaragua. "We really have emphasized a personalized response. We feel thoughtfully written letters have more impact than obviously mass campaigns."

An ACLU publication, "Tips on Writing Your Legislators," advises supporters to "keep it short, personalize it, cover only one subject, don't ramble, refer to a bill by name or number, give reasons for your position, use personal experience, know the legislator's record . . . so you can make a comment if he or she voted right on an issue."

A small number of letters on the right issue at the right time can sometimes affect policy.

In Kassebaum's first term, for example, there was such a case. Recalls press secretary Stoddard: "In late 1981 or 1982, she received several letters from people with rare diseases who complained it was impossible to get drugs to treat these diseases. That, in essense, was the genesis of the [Kassebaum-sponsored] Orphan Drug Act of 1982, which provides incentives for pharmaceutical companies to manufacture drugs for rare diseases.

"The letters were really the first time she considered it. The volume wasn't so great. It was the poignancy of the letters, the desperation, the appeal for help."

Even a single persuasive letter can have an impact, though generally short of changing a legislator's vote.

Bill Mengebier, administrative assistant to Rep. Davis cites a letter the congressman received a few years ago in response to his claim that too much land was controlled by the federal government. The constituent argued that in many cases, the forest resource on privately owned land was not being managed as well as publicly owned land. "We looked at the letter and said hmmmph, that is something we had not considered. Later, the congressman made that point part of his discussion on the subject."

According to Mengebier, on some surprising nonlegislative matters, personal appeals from constituents are frequently successful. "One woman wanted to know: Could we help her get her son home for Christmas, and in fact we did. It just illustrates the number of things people are willing to turn to us for that are not within the strict realm of what you would expect."

Most casework (requests for personal assistance) is assigned to staffers in home district offices and is nonlegislative in nature. Here, letters commonly get more individual attention.

Knowing whom to address in an office can also help. Says Serota, "On the House side, legislative assistants generally answer letters in their issue areas. If you're asking for help in resolving a casework problem, it may be a good idea to target your letter to the administrative assistant."

If you have a personal acquaintance with the person you're addressing, so much the better.

"I would drop by the home district office, get to know someone there. We did a survey of those elected in 1982, and 80 percent did all their casework in their home offices. You're likely to get face to face with the person solving your problem, or he might refer your request for you to the Washington office."

A certain diplomacy is also handy. "If I wanted to grab my member's attention," says Serota, "I would write a personal hand-written note. I'd probably start off with something complimentary, something he or she did with which I could agree, and then say, 'but I do have a bone to pick.'

"You get more responsiveness out of a congressional office when they think you're reasonable. And on the Hill, showing you're in agreement with a Congress member is a way of proving you're reasonable."