Defense lawyers for congressmen caught or convicted in the FBI's Abscam undercover operation were handed new ammunition last week to use in their fight to get their cases thrown out of court -- and it came, surprisingly, from the Justice Department.

Two department lawyers took the witness stand at the bribery and conspiracy trial of Rep. Raymond F. Lederer (D-Pa.) and disclosed that they had sternly criticized the government's star Abscam informant, convicted con man Melvin Weinberg, for trying to put incriminating words in the mouth of Sen. Harrison A. Williams Jr. (D-N.J.).

That testimony came after the department unexpectedly turned over a memorandum that sources said laid out -- and then refuted -- bitter internal criticism about the way the undercover operation was being conducted. Sources who have seen that memorandum said the prosecutors said Weinberg told them that he had to coach his targets or there would be no Abscam cases.

Significantly, the memo also disclosed some apparent inconsistencies between some of Weinberg's testimony in at least two Abscam trials and the recollection of the two federal prosecutors, these sources said.

Sources said that the inconsistencies involved Weinberg's denial that he had been criticized for his techniques or that he had been instructed about illegal entrapment.

Lawyers for four convicted congressmen are expected to assemble in a Brooklyn federal courtroom today to present evidence that the Abscam investigation was so tainted by improper conduct and so loosely supervised that it was fundamentally unfair to their clients. Judges who presided in the Abscam cases have withheld formal entry of the guilty verdicts into the court records until they have heard and evaluated these arguments.

But in Washington last week, at the bribery trial of former representative Richard Kelly (R-Fla.), the judge privately aired his thoughts on the undercover operation.

"It has an odor to it that's going to be cleared before anybody gets convicted. It has an odor to it that is absolutely repulsive," Chief Judge William B. Bryant told the lawyers, according to a court transcript.

At all the Abscam trials, defense lawyers have vigorously protested that the investigation -- and particularly Weinberg -- was poorly supervised by the Justice Department and have argued that their clients were ensnared by illegal tactics. And they have complained that the FBI -- traditionally meticulous about making a paper record of its every move -- kept no records of the Abscam investigation except for the video and audio tapes that have been the heart of the prosecutions.

Moreover, they contend, Weinberg made his own decisions about whether or not to tape telephone conversations with Abscam targets, which could mean that statements helpful to the defense are unavailable. Those lawyers have also insisted that the government has never fully disclosed its own internal concerns about Abscam, an elaborate operation in which agents masqueraded as representatives of fictitious Arab sheiks seeking help from legislators in exchange for cash.

The Justice Department, in the memo last week, downplayed the significance of its disclosures, but defense lawyers were quick to say that this new information gives additional weight to the arguments they have long made.

The developments "give us a solid evidentiary basis for things that previously we had some indications and a lot of hunches about," Michael E. Tigar, defense lawyer for former representative John M. Murphy (D-N.Y.) said over the weekend.

The information about contradictions in Weinberg's testimony could have been used to discredit Weinberg at the trials, said attorney Plato Cacheris, who represented former Philadelphia congressman Michael (Ozzie) Myers. The defense lawyers contend that the government, under a U.S. Supreme Court ruling, was obliged to supply such potentially exculpatory information for use at the Abscam trials -- but did not.

The memo is "the latest in a series of documents surfacing which show that Weinberg . . . testified falsely and people at high levels of the Justice Department knew about misconduct and withheld it until verdicts came in," said defense lawyer Kenneth M. Robinson, who represented former congressman John Jenrette (D-S.C.). "Now they do a turnabout," Robinson commented.

Jenrette, Myers, Lederer, Murphy and former representative Frank Thompson Jr. (D-N.J.) have been convicted in connection with Abscam; Kelly's trial is in progress and Williams' case is expected to come to court in February.

The government "never should have used Weinberg," said defense lawyer Richard Ben-Veniste, who represented middleman Howard L. Criden in the Abscam trials. And once it decided to use Weinberg, Ben-Veniste said, the disclosures last week show that the Justice Department's means of controlling his activities were "totally inadequate" and provided a clear opportunity for the alleged misconduct that occurred during the undercover operation.

At the U.S. District Court here, Chief Judge Bryant made his comments about the undercover operation Wednesday during one of numerous private bench conferences that have interrupted the Kelly trial. His remarks came after attorney Michael Dennis, who represents one of Kelly's co-defendants, said he planned to demand testimony from FBI agents and Weinberg "to show the nature of the corruption that pervaded this entire investigation, including the Kelly case, by Weinberg and the FBI knowingly."

"There is no question in my mind about that," Bryant responded, according to the transcript.

"Let's get along with the trial. You're anticipating something that's got to come later. But it stinks," Bryant added.