Every couple of months, Rep. Guy Vander Jagt (R-Mich.) gets a visit from Boris Davydov, a Soviet Embassy official he has known for more than eight years. They usually go around the corner from a House office building to the Republican Party-run Capitol Hill Club, where the congressman buys Davydov a drink, introduces him to other legislators and engages him in lively debate about foreign policy and other issues.

"I always thoroughly enjoy our conversations," Vander Jagt said. "He holds nothing back."

But Vander Jagt, who was the keynote speaker at the 1980 Republican National Convention, does hold something back. The day after such an encounter, Vander Jagt calls the FBI, which dispatches a counterintelligence agent to find out what Davydov wanted to discuss. U.S. intelligence files identify Davydov as an agent for the KGB, the Soviet secret intelligence service.

All over Capitol Hill, Soviet Embassy officials are collecting documents, attending hearings, talking to members of Congress and cultivating committee staff workers--all as part of an intense but rarely publicized effort to gather intelligence. Several dozen of these Soviet diplomats and embassy employes are identified in U.S. intelligence files as agents for the KGB and the GRU, the Soviet military intelligence agency.

Several congressional staff members said in interviews that they often discuss military and foreign policy with the Soviet officials, and provide them with government reports and hearing transcripts that are available to any member of the public. Others say that they refuse to talk to the Soviets or to give them press releases, and that they immediately report any attempted contact to the FBI.

The Justice Department estimates there are about 200 Soviet and Soviet-bloc intelligence agents in the nation's capital. Theodore M. Gardner, the FBI's special agent in charge of the Washington field office, said in an interview that their No. 1 target is Congress, which draws more of the KGB's resources than either the White House or the Pentagon.

With more than 18,000 staff workers, hordes of lobbyists and reporters, a constant parade of expert witnesses, reams of government documents and a porous political fabric through which most of this information invariably leaks, the Hill is a rich source for intelligence operatives on the inner workings of Washington.

While other foreign diplomats, including the British, the Israelis and the Japanese, actively engage in congressional relations on the Hill, the KGB presence there is a matter of particular concern to U.S. counterintelligence. It is not at all clear what value the Soviets place on it or exactly what they are able to get from the information gathered there.

There has been no recent public disclosure of security leaks to the KGB from the Hill, yet some legislators say the open and loose atmosphere makes it easy for the Soviets to collect a wealth of political and foreign policy information that emerges in legislative hearings and documents.

The Soviets keep a record of every person they approach in Congress and make careful assessments of how each one can be useful, the FBI's Gardner said. He said these agents are continually trying to evaluate political strategy, discover what top policymakers think, collect technical information, gain entree to well-placed officials and, ultimately, to recruit people to turn over classified information and documents.

"We believe this is their major thrust--the collection of political intelligence," Gardner said. Many congressional staff workers, he said, "become used to the presence of these people. It lowers your psychological barriers. Our people tend to be very honest and open. They do not appreciate or want to believe the ends the Soviets want to achieve.

"We see the danger because we work with it every day."

Some legislators, however, remain deeply skeptical of these charges of increased Soviet spying, saying the Reagan administration has greatly exaggerated the threat for political reasons. Some also note that foreign access to Capitol Hill actually may convince these agents that there is no great conspiracy against the Soviet Union. Other members are lining up behind legislation pending in both the House and Senate that would require all representatives of communist countries to register with the attorney general each time they want to visit anyone in Congress.

The FBI is keeping careful track of the Soviet diplomats who spend day after day on the Hill. FBI agents are authorized to conduct physical surveillance of Soviet agents, even to the point of following them through the halls of Congress.

Vander Jagt, for example, said the FBI has been interested in his meetings with Davydov since he first met the Soviet diplomat more than eight years ago at a former congressman's dinner party.

"He gives me the impression that he's just coming to see me as a friend," said Vander Jagt, who serves on the House Ways and Means Committee. "I may be a dupe, but I really believe a friendship has developed. It's like a lobbyist--you hit a spark with one another and a friendship grows.

"Maybe he's incredibly skillful. I never have the impression that he's looking for something. He's very interested in our political situation, how many seats we're going to gain and so forth. He figures it doesn't hurt to send his cables back to Moscow and say he met with me."

Vander Jagt said he believes Davydov is a KGB agent, although the two have never discussed it, and that the FBI has encouraged him to meet with Davydov more often. "I learn more than he does from these exchanges. I've never told him anything that he isn't able to read in the newspapers. I've never started to say something and then stopped and said, 'Oh my God, that's secret.' "

Davydov, who over the years has become well acquainted with many top officials and journalists, including several at The Washington Post, declined to return phone calls about this story. Several other Soviet Embassy officials also were not available for comment.

Those who argue for greater restrictions on Soviet officials here point out that American diplomats and reporters in Moscow are allowed little access to the Soviet bureaucracy. But critics of the proposed legislation, such as Rep. Don Edwards (D-Calif.), say that disclosing public policy in open hearings and reports is part of the price of maintaining a free society.

"This is the result of an administration that is fearful of foreign enemies," Edwards said. "It's mostly nonsense. We've existed for 200 years without these restrictions. I've yet to see any evidence of danger to our security on this account."

Edwards said the administration's dire warnings of a communist threat may be designed to justify U.S. covert operations in this country. "A lot of our people abroad are intelligence officers," he said. "That's perfectly appropriate. The Soviets, Cubans, English and French have them. This is the way nations operate."

Some examples of the Soviet presence on the Hill:

* On Feb. 4, FBI Director William H. Webster was telling the Senate Judiciary subcommittee on security and terrorism about the growing number of hostile spies in the United States. He said 30 to 40 percent of the roughly 600 Soviet-bloc diplomats, correspondents and trade officials here have responsibilities for gathering intelligence.

Webster then told Sen. Jeremiah Denton (R-Ala.): "It might be of interest to the chairman to know that this morning there was an official of the Russian Embassy present at these hearings."

* Last September, a Soviet diplomat walked into the office of Rep. David F. Emery (R-Maine) and asked for the congressman's public statements on his alternative plan for deploying the MX missile. When Emery's legislative aide, John Rabb, asked who he was, the man identified himself as Yuriy Petrovich Leonov. Rabb told him to leave the office and called the FBI, which showed Rabb a single photograph of Leonov and asked whether he was the visitor. U.S. intelligence files say Leonov is an agent for the GRU.

"I couldn't believe he was so brazen about it," Rabb said. "Their activities are vastly more pervasive than the average person would suspect. They offer you hockey tickets, they want to establish a relationship with you. They're very polished, very westernized, and no contact is totally innocuous."

That same month, Leonov visited the office of Rep. Larry Craig (R-Idaho), whose aides also refused to give him the congressman's statements on the MX missile, which he had made to the press that morning. In June, Leonov sat in on hearings about the MX at the Senate Armed Services subcommittee on military construction and the House Interior subcommittee on public lands. After the Emery incident was publicized, however, Leonov was sent back to the Soviet Union.

As debates over the MX, the B1 bomber and the sale of AWACS radar planes intensified last fall, Rep. Emery said he was worried because so much technical secret information was being handled in congressional offices. He arranged for the FBI to brief several dozen staff workers on what he called "a heightened intelligence effort on Capitol Hill on the part of the Soviet Embassy."

* Rep. Don Ritter (R-Pa.) said he was visited last year by Soviet Embassy counselor Sergey Chetverikov, whom intelligence files identify as a KGB agent. Ritter said he invited two staffers along and moved the meeting to the Capitol Hill Club. "I didn't want to be alone with him in my office," he said. "He was trying to cultivate me. That's his job."

* On Jan. 19, a small group of American scholars specializing in Korean affairs held an unpublicized luncheon with a State Department official at the Library of Congress. Members of the group passed around a sign-up sheet and were astonished to find that the last signature belonged to Georgiy A. Zagvozdin of the Soviet Embassy, who is named in intelligence files as a KGB agent. One of the scholars pointed out Zagvozdin and asked for his view on North Korean affairs. There was a long, embarrassed silence as Zagvozdin refused to answer.

On nearly every committee dealing with foreign or defense issues, staff members say they have come to recognize certain Soviet diplomats who attend their hearings far more faithfully than reporters. "Two or three of them seem to have nothing else to do but follow us around and gobble up whatever they can," said Michael Van Dusen, staff director of a House Foreign Affairs subcommittee. "They want to chat about policy: What's the U.S. going to do in the Middle East? How do you see the negotiations going in Geneva?

"It makes me nervous--they're popping in the door while we're in and out and there's [confidential] stuff sitting on your desk."

"They're like leeches," another House Foreign Affairs staff worker complained. "They squeeze you like a lemon for information, but they don't provide any in return. We always assume they're KGB."

One staff member who recently left a foreign policy subcommittee described how he came to have a half-dozen meetings over an 18-month period with Vladimir Vikulov, a second secretary at the Soviet Embassy.

"He would show up haphazardly, never calling to make an appointment like the other embassies do," said this former staffer, who did not want to be identified. "He always wanted documents from me, reports and hearings on U.S. foreign policy. He was very interested in the deployment of nuclear intermediate-range missiles in Europe.

"He would ask me to lunch and we would go to the Greek restaurant on Pennsylvania Avenue. It was really bizarre--he would ask where are you from, what kind of stereo system do you have, where do you go on vacation, mostly personal stuff. But he was very plodding at it, very stiff. It's like they're trying too hard.

"I would also run into him at social functions. I would give him a very hard line on foreign policy, but he would just say, 'I see, I see' and smile a lot. I couldn't get a rise out of him.

"I don't know whether he was an agent or not. It wouldn't surprise me," he said. In fact, intelligence files describe Vikulov as a member of the KGB.

"When they start asking questions about your personal life, that is the beginning of an assessment," said FBI agent Gardner. "They'd like nothing better than to have a rapport with a staffer who could give them an insight into how a particular senator might vote on SALT or disarmament."

Any single piece of information by itself may seem meaningless, Gardner said, but the Soviets are adept at assembling valuable data by pulling together seemingly unrelated items from both public and covert sources.

Some of the missing pieces may be available to the Soviets under the Freedom of Information Act, FBI officials say. The FBI has been trying to bar foreign citizens from obtaining sensitive documents under the act, which they say the Soviets have used to comb FBI files for clues on U.S. intelligence sources and methods.

The Soviets don't need the act to collect congressional documents, however. At the Senate Armed Services Committee, for example, as many as 10 Soviet Embassy and Tass officials have signed for reports and hearing transcripts. One name on nearly every list is Fedor T. Mizinov, an assistant naval attache, who intelligence sources believe is a GRU agent. In recent weeks, Mizinov has picked up copies of the 1983 defense budget and a report on U.S. military posture by the Joint Chiefs of Staff, both public documents.

"They're looking for bits and pieces," said Joel S. Lisker, staff director of Sen. Denton's security subcommittee. "Every hearing on foreign relations, trade, agriculture or international economics, the Soviets are going to cover. They're trying to see which way the decisions are going, what the thinking is."

When he went to Congress last spring, Lisker said, "I had three or four phone calls from individuals who identified themselves with Soviet organizations. They said they were very interested in the functions of the committee and wanted to have lunch." Lisker reported the calls to the FBI.

Officials recently moved to tighten security at Congress' General Accounting Office after an incident in which Soviet diplomat Vladimir Kvasov asked for a list of numbered reports on highly classified military subjects--some of which hadn't been published--and later walked around the building unescorted.

Kvasov also was involved in an earlier incident in Ely, Nev., when he and another Soviet diplomat, dressed in cowboy outfits, made copies of the government's MX missile plans at a public library while FBI agents in western garb followed them around town.

Gardner said the Soviets also have been trying to cultivate officials at the White House, federal agencies, the Library of Congress, private think tanks and consulting firms, and that they frequently approach staff members after work, at various bars, nightclubs and parties. He said the primary target remains Congress, where young staffers often are flattered by the personal attention and confident they can match wits with the KGB.

Still, some legislators do not seem overly concerned about the Soviets.

"A lot of people up here want to re-create the House Un-American Activities Committee," said Edwards. "As Jefferson told Madison, our liberties get chipped away because of our fear of enemies, whether real or perceived."