The FBI relied on convicted con men and swindlers to lead it to corrupt politicians in the Abscam investigation, even though some of those informants and middlemen had proved unreliable in the past, a high-level FBI official said yesterday.

But Francis M. Mullen Jr., executive assistant director of the FBI, assured a select Senate committee that generally the information proved reliable and that he believes the value of the Abscam investigation outweighed the problems.

"The Abscam operation presented the FBI with unprecedented leads into ongoing public corruption. If we had not gone forward with this investigation, if we had not followed our leads to their logical conclusions, the FBI would not have fulfilled its obligation to our nation," Mullen said.

The committee heard testimony from Mullen as it began the second day of hearings into the techniques used by the FBI in the controversial investigation that led to the convictions of seven members of Congress and a number of local officials who accepted bribes from FBI agents posing as representatives of wealthy Arab sheiks.

James Neal, counsel to the committee, questioned Mullen intensely on the bureau's use of convicted con man Melvin Weinberg as a central figure in the investigation.

"Here's a man who's had 25 years as a cheat, a liar. . . . Here's a man you could not control," said Neal, chief Watergate trial lawyer eight years ago. Neal established during questioning that the FBI had used Weinberg as an informant until the mid-1970s when he was dropped after the bureau learned he was conducting an independent scam on the side.

"You need a Mel Weinberg to start one of these operations," Mullen responded. "You need the instant credibility. . . . We do it all the time. We control them as best we can. We're aware going into it that they're not Boy Scouts. It's a difficult issue, but it's something we must do if we're going to succeed."

He added that because of Weinberg's background, he "was probably supervised more extensively during the course of the Abscam investigation than any other cooperating witness in the history of the FBI's undercover program."

Mullen said Weinberg received $250,000 from the FBI during the three-year Abscam investigation--which led Neal to question whether he provided information just to generate more income.

"I can state unequivocally there was no targeting of public officials," Mullen said. "There was no mention of any elected public official's name by the FBI undercover operatives prior to that name being raised by one of the corrupt influence peddlers."

Mullen was questioned sharply about authorizing bribe offers just on the word of the middlemen, even if there were no other indication the politician might be predisposed to accept a bribe. In many cases the middlemen named innocent congressmen who were weeded out later in the process.

One well-publicized case was that of Sen. Larry Pressler (R-S.D.) who was contacted to be offered a bribe. When it became clear that Pressler didn't know what was going on, the agents terminated the meeting without making the offer.

But Sen. Warren Rudman (R-N.H.), a member of the select committee, said yesterday, "Senator Pressler will carry this for life. It's almost like someone exposed to radiation. . . . In his mind, he was tainted."

Mullen acknowledged that former Camden, N.J., mayor Angelo Errichetti, who was convicted in the investigation, lied repeatedly while he worked as a middleman in the scam.

But he said that once the middlemen began bringing in congressmen who were actually accepting bribes, agents did not want to run the risk of appearing to play politics by turning down any particular lead.

"We had to be consistent. We could not deviate. Otherwise, it would have been a selective process," he said.

Sen. Walter D. Huddleston (D-Ky.) complained that he was left with the feeling that "any innocent citizen" could find himself brought before the hidden cameras of the FBI.

Mullen disagreed. "Not any citizen would have walked into Abscam, not any citizen would walk into a narcotics investigation. We're not out all over the country with television cameras seeing who we can catch," he said.