When FBI Abscam undercover agent Anthony Amoroso mentioned payoff money to Rep. Frank Thompson Jr. (D-N.J.) during a secretly videotaped meeting in Washington last year he got a sharp kick in the shins.

Thompson left the meeting without taking the cash and Amoroso then complained to Philadelphia lawyer Howard Criden: " . . . You almost broke my . . . leg you kicked me so hard." Criden, an unsuspecting middleman who brought the congressman to the FBI, retorted that Amoroso "was not supposed to discuss money in front of this guy.I made my deal with this man, okay."

It wasn't okay with the undercover agent because he wanted the congressman to acknowledge he was getting a payoff while on camera. He also told Criden he feared the lawyer might have made up a story to get Thompson to the meeting so the middleman could walk off with all the money himself.

The heated discussion that followed illustrates the kinds of problems the FBI had in dealing with the allegedly corrupt middlemen who brought public officials into the Abscam investigation. The undercover agents had to rely on the word of the middlemen that the officials were ready to accept a bribe.

The FBI also had trouble controlling the middlemen. In a later tape in the Thompson case where the cash was picked up by Criden, the lawyer suddenly asked the congressman about the chances of getting "one or two" Republicans to help the "sheik." Amoroso agreed it might be beneficial to have bipartisan support and Thompson said he was sure he knew "some good guys."

Justice Department officials insist this was not a planned attempt to target GOP members to balance the heavily Democratic flavor of Abscam.

Lawyers for Thompson and Rep. John M. Murphy (D-N.Y.), his codefendant, have argued that the two congressmen never received any of the payoff money. They allege Criden took it all.

The relationship between the undercover FBI agents and unwitting middlemen like Criden was also a consideration in the finding last week by a federal judge in Philadelphia that the Abscam conspiracy convictions of two city councilmen had to be reversed.

U.S. District Court Judge John P. Fullam said the goverment's conduct in enticing the councilmen to take payoffs was so outrageous it amounted to entrapment and a violation of their constitutional rights to due process.

In a 64-page opinion, he ruled the government was responsible for Criden's actions in the Philadelphia case. He also concluded the FBI shouldn't have relied solely on the word of the middlemen that they would bring in only persons "already corruptible."

Both Fullam's opinion and the evidence in the continuing Thompson-Murphy case raise again the most troubling of the many issues about the government's conduct in the Abscam investigation: that the FBI manufactured an opportunity for criminal activity without any real evidence that a particular official was susceptible.

The government did not hear that Thompson, Murphy or any of the other defendants was selling immigration bills before posing undercover agents as representatives of wealthy Arab businessmen. The FBI set up the scheme first to catch organized crime figures fencing stolen securities and art works. The undercover agents then shifted their attention to public officials by raising the potential immigration problems of the fictitious "sheik" and waiting to see which federal officials the middlemen would bring in.

Before Fullam's stunning opinion, the Abscam team had an unblemished string of courtroom triumphs as shocked juries handed up guilty verdicts after watching the tapes of members of Congress accepting cash payoffs. Now defendants in all the cases can hope their trial judges follow Fullam's lead and dismiss their indictments and convictions.

Though Fullam did make several sweeping statements about the general investigation, his opinion concentrated on the facts in the case against councilmen George X. Schwartz and Harry P. Jannotti, which are not all applicable to the cases against the members of Congress. For instance, he concluded that the federal government improperly manufactured jurisdiction to use a federal extortion statute to reach local officials.

Fullam also discussed the "entrapment" precedents in great detail, though the jury in the Philadelphia trial rejected it as a defense. It also was raised unsuccessfully in the Washington trial of Rep. John Jenrette (D-S.C.) and a codefendant.

U.S. District Court Judge George C. Pratt, who has the bribery cases against since-expelled Rep. Michael (Ozzie) Myers (D-Pa.), Murphy-Thompson and Sen. Harrison Williams (D-N.J.), refused to let the jury in the Myers case consider possible "due process" violations by the government. He said that was a matter of law he alone would decide.

Fullam said during the Schwartz-Jannotti trial that he expected the final word on the complex issues to be decided on appeal, so he didn't rule on them until after the trial to ensure the government's right to appeal.

The issue clearly seems headed for final resolution before the Supreme Court. If Fullam's ruling stands it will be a severe blow for the FBI.

Justice Department officials expressed confidence in congressional hearings early this year that the Abscam techniques would withstand scrutiny. They noted, for instance, that the alleged bribe transactions were so carefully monitored that supervisors in adjoining rooms would phone in instructions on legal nuances to Amoroso and other undercover agents.

Pending the appeals, Fullam's opinion is important as the first detailed judicial review of the government's conduct of the investigation. He noted, for instance, that Abscam was designed to screen out persons with legitimate business proposals for the "sheik." In practice, this wasn't followed, he said.

Angelo Errichetti, the mayor of Camden, N.J., who was convicted with Criden, a law partner, and Myers in August, gave undercover agents reasonable grounds for believing he was "thoroughly corrupt," Fullam wrote. "Mr. Errichetti's unabashed pursuit of opportunities for financial reward apparently led the undercover agents to the easy, but unjustified, assumption that anyone introduced to them by Mayor Errichetti could automatically be regarded as equally corrupt."

Melvin Weinberg, an undercover informant who worked with the FBI, also was strongly motivated to produce results, the judge said, because the government got him out a prison term, paid him well and allowed him to live in the lavish surroundings of the "sheik's" representatives.

Fullam was particularly offended by a session in which Weinberg coached Sen. Williams to boast about his importance in Washington. Williams was indicted last month on charges he agreed to accept a hidden share of a titanium mine in return for promises to seek government contracts for the metal.

Fullam wrote that Williams was seeking financing for a legitimate business deal. "Throughout the preliminary discussions, Sen. Williams made it very clear that the venture would not be dealing with the United States government, and that he neither could nor would be in any way involved with government contracts." He also said Williams "repeatedly rebuffed" suggestions from the undercover agents about ways to hide from taxes profits from resale of the mine.

"Even more disturbing" than the coaching incident, the judge added, was his finding that Thomas P. Puccio, the prosecutor who supervised Abscam, withheld that information from his superiors in a key memo.

Weinburg was never warned about the coaching, Fullam said, and the government also ignored warnings that Criden and Errichetti were out to con the FBI in another incident when an imposter unsuccessfully posed as an immigration official.

Fullam concluded that in Philadelphia, Abscam "was plainly designed, not to expose municipal corruption, not to determine which officials were corrupt, but merely to ascertain whether, given enough inducement, city officials could be corrupted."

Richard Ben-Veniste, attorney for middleman Criden, said yesterday that Fullam's opinion backs his contention that his client was unfairly ensnared by the FBI. "He came to them with a legitimate business deal. They, in effect, had a sign out saying 'Free Money' and they corrupted him."