It was billed as "Clean 87," a trade show for the nation's dry cleaners. But politics was a main attraction last April when 15,000 dry cleaners met in Atlanta. There were petitions to sign, prewritten mailgrams to members of Congress and pamphlets describing the dire consequences of a proposal to label the most common dry-cleaning fluid a probable human carcinogen.

Thus began the most intense lobbying campaign in memory involving a chemical under investigation by the Environmental Protection Agency.

Within a few days of the trade show, Congress was besieged by chemical company lobbyists, visiting delegations of dry cleaners and mailgrams by the hundreds protesting the cancer ranking for perchlorethylene (PERC) proposed by the EPA staff. Their fear, they told anyone on Capitol Hill who would listen, is that such a label would lead to job safety and air pollution controls that would be so costly they would put many dry cleaners out of business.

Among the issues of great moment in Washington, the EPA risk assessment of PERC inspires few ringing speeches or philosophical debates. Indeed, it features a side of the capital's political life usually removed from public view, the campaign of a powerful special interest to influence the outcome of an obscure regulatory decision deemed important to public health but threatening to industry fortunes.

Few industries can match the influence of the PERC proponents. As many as 25,000 of the nation's dry cleaners use the fluid to clean garments -- a powerful grass-roots force with members in virtually every congressional district. PERC, moreover, is produced by huge chemical companies with ready sources of money for political campaigns.

"There is the system, and we have to make it work for us," said Barry Stoll, administrator of the Alliance of Textile Care Associations, an industry umbrella group here that is coordinating the lobbying effort.

Nearly half the members of Congress have responded by writing EPA Administrator Lee M. Thomas of their interest in the issue -- more congressional correspondence than agency officials can recall receiving on any single issue.

Rep. Thomas A. Luken (D-Ohio), chairman of a House environmental subcommittee that oversees the EPA, wrote to Thomas in May raising questions about the staff recommendation, and his aide rounded up the signatures of seven other House members on the letter. A few weeks earlier, Luken had spoken at a seminar at the industry trade show, for which he received a $2,000 fee. A few days after the speech, he was visited in his office by industry representatives asking for his help in the PERC issue, according to Stoll.

Stoll denied any connection between the fee and the industry appeal for help. Luken failed to respond to repeated requests for an interview for this article.

Sen. John B. Breaux (D-La.) wrote Thomas at the urging of a lobbyist for Dow Chemical U.S.A., according to an aide. Dow produces PERC in Louisiana. One of its political action committees contributed $1,000 to Breaux's senatorial campaign last year.

"You don't have to talk loud and long to get the attention of a senator from Louisiana on an issue like this," said Breaux's spokesman, Bob Mann. "A lot of jobs are involved in the industry."

Said the Dow lobbyist, Frank Farfone, "I expressed our concern as a constituent."

The PERC issue surfaced in late February after the EPA's Cancer Assessment Group (CAG) recommended upgrading the chemical's status from possible to probable human carcinogen. The cancer risk assessment of a substance is important in the government's decision whether to control emissions and occupational handling.

Both the EPA and the Occupational Safety and Health Administration have imposed restrictions only on chemicals found to be known or probable human carcinogens.

PERC, which belongs to a class of chlorinated hydrocarbons linked to cancer and toxic effects in animals, was labeled a possible human carcinogen in 1985 after tests showed that mice fed the solvent developed tumors. The risk to people comes from inhaling PERC released into the general atmosphere, dry cleaning outlets and homes where dry cleaned clothing is kept.

The CAG recommendation was based on inhalation studies by the National Toxicology Program (NTP). The studies, NTP's Board of Scientific Counselors said in 1986, showed "clear evidence" of cancer-causing effects in mice and male rats and "some evidence" in female rats, said CAG Director William H. Farland.

But the NTP data was less convincing to the EPA's Science Advisory Board (SAB), an independent panel of experts who guide the administrator. Late last January, it contested some of the findings and recommended against an upgrading.

Thomas, the ultimate arbiter, is expected to weigh the SAB and CAG views. Once he determines PERC's cancer risk, another EPA unit will debate the need for pollution controls.

It is rare for lobbyists to get involved in the risk-assessment phase of a pollutant. Usually, they wait for the stage at which regulations are weighed. The normally vociferous environmental groups have been silent thus far on the PERC issue.

Stoll said the industry has focused on the risk assessment of PERC because the outcome is likely to determine the need for regulations.

An upgrading of the solvent, used by 80 percent of dry cleaners, would be "devastating," he said. Since their machines are built for specific solvents, dry cleaners cannot use substitutes in equipment designed for PERC, he said. Moreover, neither of the alternative solvents on the market is as safe as PERC, he said.

"The industry feels like it is between a rock and a hard place," noted Stoll.

By the end of March, he said, the message had spread throughout the dry cleaning world, circulated by direct mailings to the alliance's members, the trade press and word-of-mouth. They were encouraged to write, call and visit their elected representatives in Washington and ask them to pass the word to Thomas.

Luken's letter to Thomas dated May 11 noted that "a number of my constituents were upset" by reports in the trade press that the CAG opposed the advisory board's recommendation against reclassifying PERC as a probable carcinogen. He wrote that reclassifying the chemical "could bring with it a significant regulatory burden."

"The board consists of highly regarded scientists who serve as expert independent advisers to EPA," wrote Luken, "and I believe that the views of this panel ought to be accorded great weight by the agency."

Luken's subcommittee staff director, Larry Sabbath, said the "constituents" cited by Luken were members of the Cincinnati dry-cleaners' association who had written asking for his help. An earlier letter almost identical to Luken's had been sent by Breaux.

Nearly 170 million pounds of PERC were produced for dry cleaners in 1984 by factories located chiefly in Louisiana and Texas. Today's manufacturers include Dow, Occidental Petroleum Corp., PPG Industries Inc. and Vulcan Chemicals.

Most of the 240 members of Congress, including 56 senators, who have written Thomas or other EPA officials about the matter simply passed along their constituents' messages and asked for a response or an updating of the issue.

Sen. Lloyd Bentsen (D-Tex.), in a note to Thomas, pointed out the "many valid points" of a letter from a Texas dry cleaner. He sent it along with 12 identical mailgrams from other dry cleaners urging him to implore Thomas to "stand behind" the advisory board recommendation "supported by scientific data."

"No alternatives available to me," said the form mailgram. "This will put me out of business. Urgent you contact EPA."

Farland said CAG assesses the cancer-causing potential of more than 100 chemicals yearly, but none have generated as much lobbying activity as PERC. He said he has answered more than 100 congressional inquiries and dozens of telephone calls from aides. Another 300 letters or mailgrams have been sent by dry cleaners as well as a petition signed by 2,300 of them.

"Somebody did a very good job of mobilizing this group," said Farland.

But, he said, he feels no pressure from the lobbying because CAG is responsible for the scientific analysis of PERC, not its ultimate regulatory status.

"With these types of issues, I think there's room for scientific debate," he said.