Members of a Senate investigating committee yesterday accused the FBI of allowing con men, who didn't realize they were working for the FBI, to run the Abscam investigation.

Sen. Patrick J. Leahy (D-Vt.) charged that the FBI had allowed the corrupt middlemen to "take over the operation from you. . . . It seems like it's the tail wagging the dog in this case." He said the FBI allowed a corrupt middleman, Joseph Silvestri, to determine who would be brought in for possible bribe offers.

James Neal, a former Watergate prosecutor who is now the committee's chief counsel, suggested the FBI didn't do enough to prevent Silvestri and others from bringing in innocent public officials.

As the select committee went into its third day of hearings on the FBI's Abscam techniques, much of the focus turned to the case of Sen. Larry Pressler (R-S.D.). He was taken to a Washington townhouse on Nov. 7, 1979, by Silvestri, who told FBI agents Pressler would take a $50,000 bribe. Pressler was a last-minute substitute for another politician Silvestri had promised to produce.

When the agents concluded Pressler did not know what was going on, they did not offer a bribe and the meeting ended shortly afterward. Pressler had been told he was going to meet with persons interested in contributing to his campaign for president.

The controversial investigation, which involved FBI agents posing as representatives of a phony Arab sheik and promising bribes for legislative favors, led to convictions of seven members of Congress and a number of lower level officials.

John Good, an FBI agent supervisor who recommended to FBI headquarters that the meeting with Pressler take place, said, "It's a judgment call and I think we did the best we could under those circumstances . . . It took us by surprise . . . The fact that Senator Pressler came in and didn't take a bribe is the ultimate safeguard."

Good said canceling the meeting might have made Silvestri suspicious and endangered the entire operation.

"He calls me up and says he's got a higher official who'll do better for us. If we tell him, 'No, don't bring him in,' he's going to say, 'Hey, what are these guys really after' . . . If a guy comes and says he's got a bigger fish for you, how can you say 'No, I don't need him,' and still maintain credibility."

Good said the middlemen were instructed not to bring in anyone who was not prepared to accept a bribe. "We didn't want some honest congressman coming in that would blow our cover," Good said.

Committee members have complained that the FBI approved bribes to a number of innocent persons just on the recommendations of those middlemen. But Good said many people named by the con men concluded "the middleman was a sleazy individual and they didn't come in. They made the right decision."

The committee questioned whether there should be special protections for public officials targeted for undercover operations. Sen. Patrick J. Leahy (D-Vt.) said, "I'm not suggesting we're immune from prosecution," but asked if there should be "some kind of indications that would make us subject to investigation other than that we are holding public office."

Meanwhile, as a House subcommittee continued its Abscam hearings yesterday, Irvin B. Nathan, who monitored Abscam prosecutions for the Justice Department, pointed out that among the trials growing out of the investigation: "There was not a hung jury, not even a holdout . . . ."