A COUPLE of years ago, when some international trade statistics came fluttering across the desk of Republican Rep. Richard T. Schulze of Pennsylvania, alarm bells started ringing.

The figures showed that canned mushrooms from the Orient were capturing a growing share of the U.S. market. So Schulze, whose district considers itself the mushroom capital of the world, went into action: He formed a congressional mushroom caucus. The fungus forum now has 61 members from New York to California who can tell you more than you ever dare ask about the state of the American mushroom.

Earlier this year, when weekend gas station closings seemed a possibility, Democratic Rep. John W. Jenrette of South Carolina was doing the fretting. The booming Myrtle Beach resort area in his district stood to take it on the chin. No driving, no tourists, no bucks. So Jenrette created a congressional tourism caucus, which already has more than 60 members from districts where tourism is an important income-producer.

Mushroom and tourism caucuses are only part of a larger development. In fact, more than 40 of these coalitions have sprouted now on Capitol Hill. There are caucuses for women and blacks, shipyards and steel. There are caucuses for solar energy, textiles, ports, Hispanics and the Irish. There are caucuses for exports, environmentalism, coal, the Northeast, the Midwest, suburbia, rural America, metropolitan Washington, gasohol -- almost anything you can name.

In short, while members of Congress and other politicians have been complaining loudly about the growing influence of single-issue lobbying groups, the lawmakers have been engaging in their own version of the same game.

Just as single-issue lobbies are seen by many as a threat to political party unity, as a narrow and divisive force, so the spreading caucuses are evidence of the fragmentation -- some call it the Balkanization -- of the Congress. They depart from the traditional channels of committee, subcommittee, party and partisanship that long dominated Congress. Something in the traditional system seems not to have been working, particularly in an era when every answer begets two questions.

Democratic Rep. Lindy Boggs of Louisiana, a founder of the Shipyard Coalition, whose members have ship facilities back home, says that "it would certainly be helpful if we always had the overall national interest at heart," but that life in 1979 really isn't that way. As she puts it, the growth of the caucuses is a congressional reflection of "a new type of political action in the country."

Boggs adds: "More and more people are expressing their views through organizations rather, as in days gone by, when the majority of public participation was in the political parties." Help for suburbs and coal

There is no doubt that the caususes have given numerous groups a kind of clout they never wielded before, to say nothing of the political benefits for individual members when they tell constituents of the stand by this or that caucus representing their special interests in Washington.

For example, while Democratic Rep. Ronald M. Mottl of Ohio stewed for months about the problems of suburbia -- he felt federal policy was tilted against the interest of his Cleveland suburbs -- he couldn't do much about it alone.

But he formed a suburban caucus of more than 50 members with similar concerns and things began to happen. As a group, they have testified on legislation and wrangled with the executive branch. Recently they got a 40 minute meeting with President Carter and a promise of more attention to suburbs.

Another example: A caucus of senators from coal states and a counterpart group in the House have made some gains in promoting their home-state cause. The senators met with Carter and successfully pressed for relaxation of pollution-control requirements on coal-burning power generating plants.

The caucus movement may have gotten its start in the mid-1960s, when some legislators began holding ad hoc hearings as Members of Congress for peace through law (MCPL) to whip up sentiment against U.S. military involvement in Vietnam.

Other single-issue caucuses followed. A natural outgrowth of MCPL was the creation several years ago of a Vietnam veterans' caucus -- 19 members who were in military service during the Vietnam era. They push legislation to improve benefits for Vietnam veterans and try to generate more recognition their needs.

Democratic Rep. Tom Daschle of South Dakota, a member of the veterans' caucus, also happens to be the founder of another caucus, an alcohol fuels coalition whose 87 members, mostly from Midwest grain states, want to see gasohol development.

A gasohol promotion bill, drawn up by Democratic Rep. Berkley Bedell of Iowa and pushed by the caucus, recently was approved by the House Agriculture Committee and has a good chance of passage in the House.

"One joins these caucuses because the committees don't go far enough in bringing together people with the same interests or experience on the issues," Daschle says. "I believe your gasohol caucus has done that. When I came here, over 100 members had different gasohol bills, but they had no communication or coordination among themselves.

"The caucus gives us a way to find a consensus. I really look on this as part of the system, to address in an informal but organized way some of the issues we're interested in. It is a way to become informed."

Daschle's gasohol proponents meet every few weeks to learn more about the issue. Recent speakers have included Barry Commoner, the scientist, and a Brazilian government official who explained how his country spurs alcohol fuels programs.

"If you look on Congress as a formal structure, where you resolve matters in committee and on the floor, then I suppose a caucus jeopardizes that," Daschle says. "But actually the caucus enhances that. Instead of having 100 bills on, say, gasohol, the caucus can grease the legislative mill."

But make no mistake about the influence of these caucuses, benign as they might advertise themselves to be.

When Republican Rep. Paul N. McCloskey Jr. of California attempted in the House to reduce federal subsidides for the maritime industry, the Shipyard Coalition was waiting for him on the pier. With help from the Steel Caucus and its 170 additional members, the shipyard group was able to carry the day.

Ask Democratic Rep. Edward P. Beard of Rhode Island what it all means and he has a ready answer. (Beard, incidentally, disbanded his 12-member Blue Collar Caucus this year, partly because there were so few blue-collar members of Congress and partly because he got a labor subcommittee charmanship that gave him a new, official forum.)

"There's a caucus for just about everything around here, and I guess it doesn't hurt," Beard says. "You know, they teach kids in school that this is the United States. But in reality, it is a group of regions and caucuses. It's not the United States."